Bug#924527: Mention -all/non -all versions on those versions

[積丹尼 Dan Jacobson <jidanni@jidanni.org>]

Package: ffmpeg-doc
Version: 7:4.1.1-1

Kindly at the top of each of
ffmpeg: /usr/share/man/man1/ffmpeg-all.1.gz
ffmpeg: /usr/share/man/man1/ffplay-all.1.gz
ffmpeg: /usr/share/man/man1/ffprobe-all.1.gz
ffmpeg-doc: /usr/share/doc/ffmpeg/manual/ffmpeg-all.html
ffmpeg-doc: /usr/share/doc/ffmpeg/manual/ffplay-all.html
ffmpeg-doc: /usr/share/doc/ffmpeg/manual/ffprobe-all.html
be sure to mention that these are apparently actually more detailed
versions of their non "-all" counterparts.

And on their non "-all" counterparts, mention at top that there are also
the "-all" versions where more details exist.

Else users will never know the existence of the other, or think that
since on the surface they look the same, that they are some error. And
also they probably refer to /usr/bin/*-all .