string-list: note in docs that callers can specify sorting function

string-list: note in docs that callers can specify sorting function

In commit 1959bf6430 (string_list API: document what "sorted" means,
2012-09-17), Documentation/technical/api-string-list.txt was updated to
specify that strcmp() was used for sorting.  In commit 8dd5afc926
(string-list: allow case-insensitive string list, 2013-01-07), a cmp
member was added to struct string_list to allow callers to specify an
alternative comparison function, but api-string-list.txt was not
updated.  In commit 4f665f2cf3 (string-list.h: move documentation from
Documentation/api/ into header, 2017-09-26), the now out-dated
api-string-list.txt documentation was moved into string-list.h.  Update
the docs to reflect the configurability of sorting.

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