docs: fix --commit-list-format related entries

docs: fix --commit-list-format related entries

Documentation specifies that "git format-patch" would default to
format.commitListFormat if --commit-list-format is not given, but
doesn't specify the default if the format.commitListFormat is not set.
The text for --cover-letter is also obsolete as the commit list can now
be something other than a shortlog.

Document to reflect changes.

Signed-off-by: Mirko Faina <redacted>
Signed-off-by: Junio C Hamano <redacted>