fix incorrect statement in argparse docs

The documentation for the name or flags argument of `argparse.ArgumentParser.add_argument()` said

> By default, argparse automatically handles the internal naming and display names of arguments … The dest parametre defaults to the argument name with underscores `_` replacing hyphens `-` .

However, this is not true for positional arguments, which retain their hyphens.  The documentation for the dest parameter correctly describes this behavior, but a user who only reads the name or flags documentation will be misled.

Thus, I added a qualification to the sentence to make clear that the hyphen relpacement only happens for optional argiments.

Author: jkunimune

Doc/library/argparse.rst

@@ -739,9 +739,9 @@ By default, :mod:`!argparse` automatically handles the internal naming and
 display names of arguments, simplifying the process without requiring
 additional configuration.
 As such, you do not need to specify the dest_ and metavar_ parameters.
-The dest_ parameter defaults to the argument name with underscores ``_``
-replacing hyphens ``-`` . The metavar_ parameter defaults to the
-upper-cased name. For example::
+The dest_ parameter defaults to the argument name, with underscores ``_``
+replacing hyphens ``-`` if the argument is optional . The metavar_
+parameter defaults to the upper-cased name. For example::
 
    >>> parser = argparse.ArgumentParser(prog='PROG')
    >>> parser.add_argument('--foo-bar')