encoders.texi: decrease level for audio encoders section

Make Audio Encoders a separate chapter, and decrese the contained
sections level by one.

Improve rendering, and improve consistency with the other parts of the
manual.

doc/encoders.texi

@@ -17,11 +17,15 @@ with the options @code{--enable-encoder=@var{ENCODER}} /
 The option @code{-codecs} of the ff* tools will display the list of
 enabled encoders.
 
-A description of some of the currently available encoders follows.
+@c man end ENCODERS
+
+@chapter Audio Encoders
+@c man begin AUDIO ENCODERS
 
-@section Audio Encoders
+A description of some of the currently available audio encoders
+follows.
 
-@subsection ac3 and ac3_fixed
+@section ac3 and ac3_fixed
 
 AC-3 audio encoders.
 
@@ -36,7 +40,7 @@ quality audio for a given bitrate. The @var{ac3_fixed} encoder is not the
 default codec for any of the output formats, so it must be specified explicitly
 using the option @code{-acodec ac3_fixed} in order to use it.
 
-@subheading AC-3 Metadata
+@subsection AC-3 Metadata
 
 The AC-3 metadata options are used to set parameters that describe the audio,
 but in most cases do not affect the audio encoding itself. Some of the options
@@ -55,7 +59,7 @@ documents.
 @item @uref{http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/46_DDEncodingGuidelines.pdf,Dolby Digital Professional Encoding Guidelines}
 @end itemize
 
-@subsubheading Metadata Control Options
+@subsubsection Metadata Control Options
 
 @table @option
 
@@ -72,7 +76,7 @@ Metadata values can be changed before encoding each frame.
 
 @end table
 
-@subsubheading Downmix Levels
+@subsubsection Downmix Levels
 
 @table @option
 
@@ -106,7 +110,7 @@ Silence Surround Channel(s)
 
 @end table
 
-@subsubheading Audio Production Information
+@subsubsection Audio Production Information
 Audio Production Information is optional information describing the mixing
 environment.  Either none or both of the fields are written to the bitstream.
 
@@ -140,7 +144,7 @@ Small Room
 
 @end table
 
-@subsubheading Other Metadata Options
+@subsubsection Other Metadata Options
 
 @table @option
 
@@ -195,7 +199,7 @@ Original Source (default)
 
 @end table
 
-@subsubheading Extended Bitstream Information
+@subsection Extended Bitstream Information
 The extended bitstream options are part of the Alternate Bit Stream Syntax as
 specified in Annex D of the A/52:2010 standard. It is grouped into 2 parts.
 If any one parameter in a group is specified, all values in that group will be
@@ -205,7 +209,7 @@ will use these values instead of the ones specified in the @code{center_mixlev}
 and @code{surround_mixlev} options if it supports the Alternate Bit Stream
 Syntax.
 
-@subsubheading Extended Bitstream Information - Part 1
+@subsubsection Extended Bitstream Information - Part 1
 
 @table @option
 
@@ -302,7 +306,7 @@ Silence Surround Channel(s)
 
 @end table
 
-@subsubheading Extended Bitstream Information - Part 2
+@subsubsection Extended Bitstream Information - Part 2
 
 @table @option
 
@@ -353,7 +357,7 @@ HDCD A/D Converter
 
 @end table
 
-@subheading Other AC-3 Encoding Options
+@subsection Other AC-3 Encoding Options
 
 @table @option
 
@@ -365,4 +369,4 @@ is highly recommended that it be left as enabled except for testing purposes.
 
 @end table
 
-@c man end ENCODERS
+@c man end AUDIO ENCODERS