Better wording.

httpie/cli.py

@@ -126,7 +126,7 @@ parser.add_argument(
     '--auth', '-a', type=cliparse.AuthCredentialsType(cliparse.SEP_COMMON),
     help=_('''
         username:password.
-        If the password is omitted (-a username), HTTPie will prompt for it.
+        If only the username is provided (-a username), HTTPie will prompt for the password.
     '''),
 )
 

httpie/cliparse.py

@@ -230,6 +230,7 @@ class AuthCredentials(KeyValue):
 
     """
     def _getpass(self, prompt):
+        # To allow mocking.
         return getpass.getpass(prompt)
 
     def has_password(self):