string_list API: document what "sorted" means

The recent work on using string_list to represent the list of refs
that matched with the refs on the other side during fetch heavily
depends on the sort order by string_list's implementation, and
changing string_list will break it.  Document that it uses strcmp()
order, at least for now.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-string-list.txt

@@ -1,8 +1,9 @@
 string-list API
 ===============
 
-The string_list API offers a data structure and functions to handle sorted
-and unsorted string lists.
+The string_list API offers a data structure and functions to handle
+sorted and unsorted string lists.  A "sorted" list is one whose
+entries are sorted by string value in `strcmp()` order.
 
 The 'string_list' struct used to be called 'path_list', but was renamed
 because it is not specific to paths.
@@ -143,7 +144,7 @@ write `string_list_insert(...)->util = ...;`.
 
 `sort_string_list`::
 
-	Make an unsorted list sorted.
+	Sort the list's entries by string value in `strcmp()` order.
 
 `unsorted_string_list_has_string`::