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

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