gh-65169: Clarify prog default in argparse (GH-31602)

Author: slateny

Doc/library/argparse.rst

@@ -249,11 +249,12 @@ The following sections describe how each of these are used.
 prog
 ^^^^
 
-By default, :class:`ArgumentParser` objects use ``sys.argv[0]`` to determine
+By default, :class:`ArgumentParser` objects use the base name
+(see :func:`os.path.basename`) of ``sys.argv[0]`` to determine
 how to display the name of the program in help messages.  This default is almost
-always desirable because it will make the help messages match how the program was
-invoked on the command line.  For example, consider a file named
-``myprogram.py`` with the following code::
+always desirable because it will make the help messages match the name that was
+used to invoke the program on the command line.  For example, consider a file
+named ``myprogram.py`` with the following code::
 
    import argparse
    parser = argparse.ArgumentParser()