doc: move syntax and eval chapters from libavutil to ffmpeg-utils

Move utilities description to a dedicated manual, consistent with what I
did with the rest of the documentation.

doc/ffmpeg-utils.texi

@@ -0,0 +1,38 @@
+\input texinfo @c -*- texinfo -*-
+
+@settitle FFmpeg Utilities Documentation
+@titlepage
+@center @titlefont{FFmpeg Utilities Documentation}
+@end titlepage
+
+@top
+
+@contents
+
+@chapter Description
+@c man begin DESCRIPTION
+
+This document describes some generic features and utilities provided
+by the libavutil library.
+
+@c man end DESCRIPTION
+
+@include syntax.texi
+@include eval.texi
+
+@ignore
+
+@setfilename ffmpeg-utils
+@settitle FFmpeg utilities
+
+@c man begin SEEALSO
+ffmpeg(1), ffplay(1), ffprobe(1), ffserver(1), libavutil(3)
+@c man end
+
+@c man begin AUTHORS
+See Git history (git://source.ffmpeg.org/ffmpeg)
+@c man end
+
+@end ignore
+
+@bye