bpo-21910: Clarify docs for codecs writelines method (GH-31245)

Co-authored-by: Jelle Zijlstra <[email protected]>
(cherry picked from commit a8c87a2)

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

Author: miss-islington

Doc/library/codecs.rst

@@ -694,8 +694,9 @@ compatible with the Python codec registry.
 
    .. method:: writelines(list)
 
-      Writes the concatenated list of strings to the stream (possibly by reusing
-      the :meth:`write` method). The standard bytes-to-bytes codecs
+      Writes the concatenated iterable of strings to the stream (possibly by reusing
+      the :meth:`write` method). Infinite or
+      very large iterables are not supported. The standard bytes-to-bytes codecs
       do not support this method.