Issue #9343: Document that argparse parent parsers must be configured before their children. (Merge from 3.2.)

Doc/library/argparse.rst

@@ -351,6 +351,11 @@ Note that most parent parsers will specify ``add_help=False``.  Otherwise, the
 :class:`ArgumentParser` will see two ``-h/--help`` options (one in the parent
 and one in the child) and raise an error.
 
+.. note::
+   You must fully initialize the parsers before passing them via ``parents=``.
+   If you change the parent parsers after the child parser, those changes will
+   not be reflected in the child.
+
 
 formatter_class
 ^^^^^^^^^^^^^^^

Misc/NEWS

@@ -304,6 +304,9 @@ Library
 
 - Issue #8982: Improve the documentation for the argparse Namespace object.
 
+- Issue #9343: Document that argparse parent parsers must be configured before
+  their children.
+
 Build
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