Replace @node commands with @anchor to refer to specific points in the text.

This fixes the warnings
** `Developers Guide' is up for `Coding Rules', but has no menu entry for this node
** `Invocation' is up for `FFmpeg formula evaluator', but has no menu entry for this node

Originally committed as revision 19831 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/developer.texi

@@ -33,7 +33,7 @@ You can use libavcodec or libavformat in your commercial program, but
 @emph{any patch you make must be published}. The best way to proceed is
 to send your patches to the FFmpeg mailing list.
 
-@node Coding Rules
+@anchor{Coding Rules}
 @section Coding Rules
 
 FFmpeg is programmed in the ISO C90 language with a few additional

doc/ffmpeg-doc.texi

@@ -826,7 +826,7 @@ absolute pathname, ffmpeg will search for that filename. This way you
 can specify the absolute and complete filename of the preset file, for
 example @file{./ffpresets/libx264-max.ffpreset}.
 
-@node FFmpeg formula evaluator
+@anchor{FFmpeg formula evaluator}
 @section FFmpeg formula evaluator
 
 When evaluating a rate control string, FFmpeg uses an internal formula