Update bash completion header documentation

Update bash completion header documentation

 1) Added a note about supporting the long options for most commands,
    as we have been doing so for quite some time.

 2) Include a notice that these routines are covered by the GPL,
    as that may not be obvious, even though they are distributed
    as part of the core Git distribution.

 3) Added a short section on how to send patches to the routines,
    and to whom they should get sent to.  Currently that is me,
    as I am the active maintainer.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>