[3.12] gh-121196: Document dict.fromkeys params as pos-only (GH-121197) (#121243)

gh-121196: Document `dict.fromkeys` params as pos-only (GH-121197)
(cherry picked from commit 1dc9a4f)

Co-authored-by: sobolevn <[email protected]>

Author: miss-islington

Doc/library/stdtypes.rst

@@ -4556,7 +4556,7 @@ can be used interchangeably to index the same dictionary entry.
 
       Return a shallow copy of the dictionary.
 
-   .. classmethod:: fromkeys(iterable, value=None)
+   .. classmethod:: fromkeys(iterable, value=None, /)
 
       Create a new dictionary with keys from *iterable* and values set to *value*.