Further nitpicking on modify --threeipd help

- State usage information first thing; Description of what it actually is,
  state last.
- Remove information of clearing in perparation of separate --clear-threepids
  option.
- Explicitely state that existing users will get all threepids replaced!

synadm/cli/user.py

@@ -394,14 +394,15 @@ def name_extra(self):
     help="Set display name. defaults to the value of user_id")
 @optgroup.option(
     "--threepid", "-t", type=str, multiple=True, nargs=2,
-    help="""Add a third-party identifier (email address or phone number).
-    Threepids are used for several things: For use when logging in, as an
-    alternative to the user id; in the case of email, as an alternative contact
-    to help with account recovery; as well as to receive notifications of
-    missed messages. This option requires two arguments: `medium value` (eg.
-    `--threepid email <user@example.org>`). To configure multiple entries for a
-    user, this option can be passed multiple times. To clear _all_ threepids,
-    pass it _once_ while setting two empty strings: '' ''.""")
+    help="""Set a third-party identifier (email address or phone number). Pass
+    two arguments: `medium value` (eg. `--threepid email <user@example.org>`).
+    This option can be passed multiple times, which allows setting multiple
+    entries for a user. When modifying existing users, all threepids are
+    replaced by what's passed in all given `--threepid` options. Threepids are
+    used for several things: For use when logging in, as an alternative to the
+    user id; in the case of email, as an alternative contact to help with
+    account recovery, as well as to receive notifications of missed
+    messages.""")
 @optgroup.option(
     "--avatar-url", "-v", type=str,
     help="""Set avatar URL. Must be a MXC URI