replace FFmpeg with Libav in doc/
unless it stands for ffmpeg the program or is historic
Documentation updates for the git migration
This cleanup patch updates the developer documentation with respect tothe migration to the git scm.
Originally committed as revision 5942 to svn://svn.ffmpeg.org/ffmpeg/trunk
things to take care of for the next soc
Originally committed as revision 5941 to svn://svn.ffmpeg.org/ffmpeg/trunk