git-for-each-ref: improve the documentation on scripting modes

git-for-each-ref: improve the documentation on scripting modes

When reading the synopsis for git-for-each-ref it is easy to miss
the obvious power of --shell and family.  Call this feature out in
the primary paragragh.  Also add more description to the examples
to indicate which features we are demonstrating.  Finally add a
very simple eval based example in addition to the very complex one
to give a gentler introduction.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>