gh-135623: Document that json sorts the keys before coercing them to strings (GH-156987)

(cherry picked from commit 6474041)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: 2 people

Doc/library/json.rst

@@ -264,6 +264,8 @@ Basic Usage
       into JSON and then back into a dictionary, the dictionary may not equal
       the original one. That is, ``loads(dumps(x)) != x`` if x has non-string
       keys.
+      *sort_keys* sorts the keys before they are coerced to strings,
+      so numeric keys are sorted by value, not by their string representation.
 
 .. function:: load(fp, *, cls=None, object_hook=None, parse_float=None, \
                    parse_int=None, parse_constant=None, \