doc: clarify documentation for rename/copy limits

doc: clarify documentation for rename/copy limits

A few places in the docs implied that rename/copy detection is always
quadratic or that all (unpaired) files were involved in the quadratic
portion of rename/copy detection.  The following two commits each
introduced an exception to this:

    9027f53cb505 (Do linear-time/space rename logic for exact renames,
                  2007-10-25)
    bd24aa2f97a0 (diffcore-rename: guide inexact rename detection based
                  on basenames, 2021-02-14)

(As a side note, for copy detection, the basename guided inexact rename
detection is turned off and the exact renames will only result in
sources (without the dests) being removed from the set of files used in
quadratic detection.  So, for copy detection, the documentation was
closer to correct.)

Avoid implying that all files involved in rename/copy detection are
subject to the full quadratic algorithm.  While at it, also note the
default values for all these settings.

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