doc: add libavutil.texi

With some fixes and better wording by Alexander Strasser
<eclipse7@gmx.net>.

doc/Makefile

+DOCLIBS-$(CONFIG_AVUTIL)  += libavutil
 DOCLIBS-$(CONFIG_AVCODEC) += libavcodec
 
 MANPAGES    = $(PROGS-yes:%=doc/%.1)    $(DOCLIBS-yes:%=doc/%.3)

doc/libavcodec.texi

@@ -1116,7 +1116,7 @@ Set sample format audio decoders should prefer. Default value is
 @settitle media streams decoding and encoding library
 
 @c man begin SEEALSO
-ffmpeg(1), ffplay(1), ffprobe(1), ffserver(1)
+ffmpeg(1), ffplay(1), ffprobe(1), ffserver(1), libavutil(3)
 @c man end
 
 @c man begin AUTHORS

doc/libavutil.texi

+\input texinfo @c -*- texinfo -*-
+
+@settitle Libavutil Documentation
+@titlepage
+@center @titlefont{Libavutil Documentation}
+@end titlepage
+
+@top
+
+@contents
+
+@chapter Description
+@c man begin DESCRIPTION
+
+The libavutil library is a utility library to aid portable
+multimedia programming. It contains safe portable string functions,
+random number generators, data structures, additional mathematics
+functions, cryptography and multimedia related functionality (like
+enumerations for pixel and sample formats).
+
+@c man end DESCRIPTION
+
+@include syntax.texi
+@include eval.texi
+
+@ignore
+
+@setfilename libavutil
+@settitle multimedia-biased utility library
+
+@c man begin SEEALSO
+ffmpeg(1), ffplay(1), ffprobe(1), ffserver(1)
+@c man end
+
+@c man begin AUTHORS
+See Git history (git://source.ffmpeg.org/ffmpeg)
+@c man end
+
+@end ignore
+
+@bye