doc: correct misleading descriptions for --shallow-exclude

doc: correct misleading descriptions for --shallow-exclude

The documentation for the --shallow-exclude option to clone/fetch/etc.
claims that the option takes a revision, but it does not.  As per
upload-pack.c's process_deepen_not(), it passes the option to
expand_ref() and dies if it does not find exactly one ref matching the
name passed.  Further, this has always been the case ever since these
options were introduced by the commits merged in a460ea4a3cb1 (Merge
branch 'nd/shallow-deepen', 2016-10-10).  Fix the documentation to
match the implementation.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>