Mention remote debugging via -p PID in usage text

Adds a brief note to the pdb help summary about attaching to a running
process using the -p option, making the remote debugging feature
more visible.

Author: ivonastojanovic

Lib/pdb.py

@@ -3489,7 +3489,8 @@ def help():
 _usage = """\
 Debug the Python program given by pyfile. Alternatively,
 an executable module or package to debug can be specified using
-the -m switch.
+the -m switch. You can also attach to a running Python process
+using the -p option with its PID.
 
 Initial commands are read from .pdbrc files in your home directory
 and in the current directory, if they exist.  Commands supplied with