Rename the ff* tools @chapters "Invocation" to "Synopsis", and place

them at the beginning of the file, for consistency with the final
ordering of the corresponding man pages.

Also create a separate @chapter for the "Options" section, as it is
done for the man pages, rather than merge it in the same section of
"Synopsis".

Originally committed as revision 23541 to svn://svn.ffmpeg.org/ffmpeg/trunk