doc: add libavfilter.texi

With some re-wording by Alexander Strasser <eclipse7@gmx.net>.

doc/Makefile

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

doc/libavfilter.texi

+\input texinfo @c -*- texinfo -*-
+
+@settitle Libavfilter Documentation
+@titlepage
+@center @titlefont{Libavfilter Documentation}
+@end titlepage
+
+@top
+
+@contents
+
+@chapter Description
+@c man begin DESCRIPTION
+
+The libavfilter library provides a generic audio/video filtering
+framework containing several filters, sources and sinks.
+
+@c man end DESCRIPTION
+
+@include filters.texi
+
+@ignore
+
+@setfilename libavfilter
+@settitle multimedia filtering library
+
+@c man begin SEEALSO
+ffmpeg(1), ffplay(1), ffprobe(1), ffserver(1), libavutil(3), libavcodec(3)
+@c man end
+
+@c man begin AUTHORS
+See Git history (git://source.ffmpeg.org/ffmpeg)
+@c man end
+
+@end ignore
+
+@bye