worktree: add can be created from any commit-ish

worktree: add can be created from any commit-ish

Currently 'git worktree add' is documented to take an optional <branch>
argument, which is checked out in the new worktree.  However it is more
generally possible to use a commit-ish as the optional argument, and
check that out into the new worktree.

Document that this is a possibility, as new users of git worktree add
might find it helpful.

Reported-by: Randall S. Becker <rsbecker@nexbridge.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>