doc: set documentencoding on toplevel texi files.

Fix double UTF-8 encoding on output files.
makeinfo / texi2any does not seem to provide a way of setting
it globally for toplevel files.

doc/developer.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle Developer Documentation
 @titlepage

doc/faq.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle FFmpeg FAQ
 @titlepage

doc/fate.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle FFmpeg Automated Testing Environment
 @titlepage

doc/ffmpeg-bitstream-filters.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle FFmpeg Bitstream Filters Documentation
 @titlepage

doc/ffmpeg-codecs.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle FFmpeg Codecs Documentation
 @titlepage

doc/ffmpeg-devices.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle FFmpeg Devices Documentation
 @titlepage

doc/ffmpeg-filters.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle FFmpeg Filters Documentation
 @titlepage

doc/ffmpeg-formats.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle FFmpeg Formats Documentation
 @titlepage

doc/ffmpeg-protocols.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle FFmpeg Protocols Documentation
 @titlepage

doc/ffmpeg-resampler.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle FFmpeg Resampler Documentation
 @titlepage

doc/ffmpeg-scaler.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle FFmpeg Scaler Documentation
 @titlepage

doc/ffmpeg-utils.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle FFmpeg Utilities Documentation
 @titlepage

doc/ffmpeg.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle ffmpeg Documentation
 @titlepage

doc/ffplay.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle ffplay Documentation
 @titlepage

doc/ffprobe.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle ffprobe Documentation
 @titlepage

doc/ffserver.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle ffserver Documentation
 @titlepage

doc/general.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle General Documentation
 @titlepage

doc/git-howto.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle Using git to develop FFmpeg
 

doc/libavcodec.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle Libavcodec Documentation
 @titlepage

doc/libavdevice.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle Libavdevice Documentation
 @titlepage

doc/libavfilter.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle Libavfilter Documentation
 @titlepage

doc/libavformat.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle Libavformat Documentation
 @titlepage

doc/libavutil.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle Libavutil Documentation
 @titlepage

doc/libswresample.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle Libswresample Documentation
 @titlepage

doc/libswscale.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle Libswscale Documentation
 @titlepage

doc/nut.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle NUT
 

doc/platform.texi

 \input texinfo @c -*- texinfo -*-
+@documentencoding UTF-8
 
 @settitle Platform Specific Information
 @titlepage