Documentation/git-status: mention how to detect copies

Documentation/git-status: mention how to detect copies

The man page documents that git-status can find copies, but does not
mention how. Whereas git-diff has command line options -C, there is
no such option for git-status - it will only detect copies when the
"status.renames" config option is "copies" or "copy". Document that
in git-status.txt because this has confused me and others[1].

[1]: https://www.reddit.com/r/git/comments/ppc2l9/how_to_get_a_file_with_copied_status/

Signed-off-by: Johannes Altmanninger <aclopte@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>