[Doc] Update child users section in user_types.md

The three main types of child accounts are Unicorn, Geller, and
Griffin. Child account should be reserved as the umbrella term that
refers to all supervised users.

Bug: None
Change-Id: I266aa26c5b41c812ed9f86fa9f4f8538ff11761b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2459397Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Auto-Submit: Toby Huang <tobyhuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822003}

docs/login/user_types.md

@@ -16,9 +16,12 @@ Regular users that were registered using their GAIA account.
 ## Child users
 
 Users that logged in using
-*   a child account - an account designated for children under the age of 13.
+*   a Unicorn account - an account designated for children under the age of
+    consent in their jurisdiction.
 *   a Geller account - an account with parental supervision that has no age
     restrictions.
+*   a Griffin account - similar to a Geller account, but for compliance with
+    European Union laws.
 
 In order to add a child user to the device, the user has to go through an
 adapted GAIA flow, which also requires their parent to authenticate.