doc/codecs: Change common boolean parameters listed to "bool"

Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/codecs.texi

@@ -1258,7 +1258,7 @@ Interlaced video, top coded first, bottom displayed first
 Interlaced video, bottom coded first, top displayed first
 @end table
 
-@item skip_alpha @var{integer} (@emph{decoding,video})
+@item skip_alpha @var{bool} (@emph{decoding,video})
 Set to 1 to disable processing alpha (transparency). This works like the
 @samp{gray} flag in the @option{flags} option which skips chroma information
 instead of alpha. Default is 0.