[5.2.x] Fixes #36215 -- Included unpacking generalization notes in coding style guide (PEP-448).

Backport of 6b3250673937b105af44f2f14247e56876f8dbe1 from main.

docs/internals/contributing/writing-code/coding-style.txt

@@ -127,6 +127,10 @@ Python style
         """
         ...
 
+* Where applicable, use unpacking generalizations compliant with :pep:`448`,
+  such as merging mappings (``{**x, **y}``) or sequences (``[*a, *b]``). This
+  improves performance, readability, and maintainability while reducing errors.
+
 .. _coding-style-imports:
 
 Imports