bpo-31330: Clarify that RawTextHelpFormatter collapses repeated newlines. (#3272)

Also provide a solution if the user wants to keep multiple blank lines.

Doc/library/argparse.rst

@@ -426,7 +426,9 @@ should not be line-wrapped::
     -h, --help  show this help message and exit
 
 :class:`RawTextHelpFormatter` maintains whitespace for all sorts of help text,
-including argument descriptions.
+including argument descriptions. However, multiple new lines are replaced with
+one. If you wish to preserve multiple blank lines, add spaces between the
+newlines.
 
 :class:`ArgumentDefaultsHelpFormatter` automatically adds information about
 default values to each of the argument help messages::