doc/general.texi: Adjust the notes on AviSynth

FFmpeg provides local copies of these headers in compat/avisynth/,
and there is no restriction against using 2.5.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 5336cd63)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/general.texi

@@ -164,30 +164,17 @@ you must upgrade FFmpeg's license to GPL in order to use it.
 
 @section AviSynth
 
-FFmpeg can read AviSynth scripts as input. To enable support you need a
-suitable @file{avisynth_c.h} header to compile against. The header in
-classic AviSynth's CVS repository is not compatible as it has not been
-updated to support AviSynth 2.6. AviSynth 2.5 is not supported by FFmpeg.
-Once you have the appropriate header, pass @code{--enable-avisynth} to
-configure to enable AviSynth support.
+FFmpeg can read AviSynth scripts as input. To enable support, pass
+@code{--enable-avisynth} to configure.  The correct headers are
+included in compat/avisynth/, which allows the user to enable support
+without needing to search for these headers themselves.
 
 For Windows, supported AviSynth variants are
-@url{http://avisynth.nl, AviSynth 2.6} for 32-bit builds and
+@url{http://avisynth.nl, AviSynth 2.5 or 2.6} for 32-bit builds and
 @url{http://avs-plus.net, AviSynth+ 0.1} for 32-bit and 64-bit builds.
-The necessary @file{avisynth_c.h} header is the variant in the @file{extras/}
-directory of the @url{https://www.videolan.org/developers/x264.html, x264}
-source tree. For convenience, this header is also available from a
-@url{https://github.com/qyot27/avisynth_headers, temporary repository}
-along with an installation routine.
 
 For Linux and OS X, the supported AviSynth variant is
 @url{https://github.com/avxsynth/avxsynth, AvxSynth}.
-@file{avxsynth_c.h} is installed as part of the normal
-build routine, as illustrated on
-@url{https://github.com/avxsynth/avxsynth/wiki/System-Setup, AvxSynth's wiki}.
-(the instructions for compiling its prerequisites are outdated, as FFMS 2.18
-or higher is now needed; the list of dependencies to be downloaded from the
-repositories is still the same, though).
 
 @float NOTE
 AviSynth and AvxSynth are loaded dynamically.  Distributors can build FFmpeg