for-each-ref: clean up documentation of --format

for-each-ref: clean up documentation of --format

Move the description of the `*` prefix from the --format option
documentation to the part of the command documentation that deals with other
object type-specific modifiers. Also reorganize and reword the remaining
--format documentation so that the explanation of the default format doesn't
interrupt the details on format string interpolation.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>