gh-109292: add symtable impact of PEP 709 to What's New (#109293)

Author: carljm

Doc/whatsnew/3.12.rst

@@ -257,6 +257,9 @@ Inlining does result in a few visible behavior changes:
 
 * There is no longer a separate frame for the comprehension in tracebacks,
   and tracing/profiling no longer shows the comprehension as a function call.
+* The :mod:`symtable` module will no longer produce child symbol tables for each
+  comprehension; instead, the comprehension's locals will be included in the
+  parent function's symbol table.
 * Calling :func:`locals` inside a comprehension now includes variables
   from outside the comprehension, and no longer includes the synthetic ``.0``
   variable for the comprehension "argument".