Documentation: use proper wording for ref format strings

Documentation: use proper wording for ref format strings

Various commands list refs and allow to use a format string for the
output that interpolates from the ref as well as the object it points
at (for-each-ref; branch and tag in list mode).

Currently, the documentation talks about interpolating from the object.
This is confusing because a ref points to an object but not vice versa,
so the object cannot possible know %(refname), for example. Thus, this is
wrong independent of refs being objects (one day, maybe) or not.

Change the wording to make this clearer (and distinguish it from formats
for the log family).

Signed-off-by: Michael J Gruber <git@grubix.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>