doc/filters.texi: fix time duration references

Make time duration references consistent, using @ref links, and
eliminate incorrect syntax [-]HH[:MM[:SS[.m...]]].
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>

doc/filters.texi

@@ -517,27 +517,21 @@ volume as the input audio, at the end of the fade-out transition
 the output audio will be silence. Default is 44100.
 
 @item start_time, st
-Specify time for starting to apply the fade effect. Default is 0.
-The accepted syntax is:
-@example
-[-]HH[:MM[:SS[.m...]]]
-[-]S+[.m...]
-@end example
-See also the function @code{av_parse_time()}.
-If set this option is used instead of @var{start_sample} one.
+Specify the start time of the fade effect. Default is 0.
+The value must be specified as a time duration; see
+@ref{time duration syntax,,the Time duration section in the ffmpeg-utils(1) manual,ffmpeg-utils}
+for the accepted syntax.
+If set this option is used instead of @var{start_sample}.
 
 @item duration, d
-Specify the duration for which the fade effect has to last. Default is 0.
-The accepted syntax is:
-@example
-[-]HH[:MM[:SS[.m...]]]
-[-]S+[.m...]
-@end example
-See also the function @code{av_parse_time()}.
+Specify the duration of the fade effect. See
+@ref{time duration syntax,,the Time duration section in the ffmpeg-utils(1) manual,ffmpeg-utils}
+for the accepted syntax.
 At the end of the fade-in effect the output audio will have the same
 volume as the input audio, at the end of the fade-out transition
 the output audio will be silence.
-If set this option is used instead of @var{nb_samples} one.
+By default the duration is determined by @var{nb_samples}.
+If set this option is used instead of @var{nb_samples}.
 
 @item curve
 Set curve for fade transition.
@@ -1120,9 +1114,9 @@ The number of the first sample that should be output.
 The number of the first sample that should be dropped.
 @end table
 
-@option{start}, @option{end}, @option{duration} are expressed as time
-duration specifications, check the "Time duration" section in the
-ffmpeg-utils manual.
+@option{start}, @option{end}, and @option{duration} are expressed as time
+duration specifications; see
+@ref{time duration syntax,,the Time duration section in the ffmpeg-utils(1) manual,ffmpeg-utils}.
 
 Note that the first two sets of the start/end options and the @option{duration}
 option look at the frame timestamp, while the _sample options simply count the
@@ -1637,9 +1631,9 @@ Set the number of samples per channel per each output frame, default
 is 1024. Only used if plugin have zero inputs.
 
 @item duration, d
-Set the minimum duration of the sourced audio. See the function
-@code{av_parse_time()} for the accepted format, also check the "Time duration"
-section in the ffmpeg-utils manual.
+Set the minimum duration of the sourced audio. See
+@ref{time duration syntax,,the Time duration section in the ffmpeg-utils(1) manual,ffmpeg-utils}
+for the accepted syntax.
 Note that the resulting duration may be greater than the specified duration,
 as the generated audio is always cut at the end of a complete frame.
 If not specified, or the expressed duration is negative, the audio is
@@ -2189,8 +2183,9 @@ Set the channel layout. The number of channels in the specified layout
 must be equal to the number of specified expressions.
 
 @item duration, d
-Set the minimum duration of the sourced audio. See the function
-@code{av_parse_time()} for the accepted format.
+Set the minimum duration of the sourced audio. See
+@ref{time duration syntax,,the Time duration section in the ffmpeg-utils(1) manual,ffmpeg-utils}
+for the accepted syntax.
 Note that the resulting duration may be greater than the specified
 duration, as the generated audio is always cut at the end of a
 complete frame.
@@ -8546,9 +8541,10 @@ The number of the first frame that should be passed to the output.
 The number of the first frame that should be dropped.
 @end table
 
-@option{start}, @option{end}, @option{duration} are expressed as time
-duration specifications, check the "Time duration" section in the
-ffmpeg-utils manual.
+@option{start}, @option{end}, and @option{duration} are expressed as time
+duration specifications; see
+@ref{time duration syntax,,the Time duration section in the ffmpeg-utils(1) manual,ffmpeg-utils}
+for the accepted syntax.
 
 Note that the first two sets of the start/end options and the @option{duration}
 option look at the frame timestamp, while the _frame variants simply count the
@@ -9459,12 +9455,9 @@ number or a valid video frame rate abbreviation. The default value is
 "25".
 
 @item duration, d
-Set the video duration of the sourced video. The accepted syntax is:
-@example
-[-]HH:MM:SS[.m...]
-[-]S+[.m...]
-@end example
-See also the function @code{av_parse_time()}.
+Set the duration of the sourced video. See
+@ref{time duration syntax,,the Time duration section in the ffmpeg-utils(1) manual,ffmpeg-utils}
+for the accepted syntax.
 
 If not specified, or the expressed duration is negative, the video is
 supposed to be generated forever.
@@ -9726,12 +9719,9 @@ number or a valid video frame rate abbreviation. The default value is
 Set the sample aspect ratio of the sourced video.
 
 @item duration, d
-Set the video duration of the sourced video. The accepted syntax is:
-@example
-[-]HH[:MM[:SS[.m...]]]
-[-]S+[.m...]
-@end example
-Also see the the @code{av_parse_time()} function.
+Set the duration of the sourced video. See
+@ref{time duration syntax,,the Time duration section in the ffmpeg-utils(1) manual,ffmpeg-utils}
+for the accepted syntax.
 
 If not specified, or the expressed duration is negative, the video is
 supposed to be generated forever.