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>

string-list.h

@@ -4,7 +4,8 @@
 /**
  * The string_list API offers a data structure and functions to handle
  * sorted and unsorted arrays of strings.  A "sorted" list is one whose
- * entries are sorted by string value in `strcmp()` order.
+ * entries are sorted by string value in the order specified by the `cmp`
+ * member (`strcmp()` by default).
  *
  * The caller:
  *
@@ -209,7 +210,8 @@ struct string_list_item *string_list_append(struct string_list *list, const char
 struct string_list_item *string_list_append_nodup(struct string_list *list, char *string);
 
 /**
- * Sort the list's entries by string value in `strcmp()` order.
+ * Sort the list's entries by string value in order specified by list->cmp
+ * (strcmp() if list->cmp is NULL).
  */
 void string_list_sort(struct string_list *list);