[bug-cflow] manual doesn't mention that --format=posix implies what --brief does to --format=gnu

[Britton Kerin]

I switched from --format=posix to --format=gnu and got a huge (1M
line) file.  On investigation,
it seems that --format=posix effectively implies what --brief normaly
does to --format=gnu.
It might be worth explicitly mentioning this in the manual.

Currently the manual only mentions --brief in the context of
--reverse, and the example POSIX format output in Chapter 4 Variout
Output Formats doesn' t show a case where --brief is relevant.

It might also be better to describe --brief for the first time
somewhere that doesn't subtly imply
that it is specific to --reverse.  A little thought shows that this
isn't the case but the manual
makes it look a little like it might be.

Britton