Mention LoggedInUserMixin in user_types.md documentation

This documentation-only CL modifies user_types.md to mention
LoggedInUserMixin as a convenient way to log in as child user for
browser tests.

Bug: None
Change-Id: Ib6d3833600bb1fcd74e1fee61e0d68446dbee7c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1911219Reviewed-by: Aga Wronska <agawronska@chromium.org>
Commit-Queue: Toby Huang <tobyhuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714524}

docs/login/user_types.md

@@ -13,7 +13,6 @@ enterprise user types, see
 
 Regular users that were registered using their GAIA account.
 
-
 ## Child users
 
 Users that logged in using
@@ -52,6 +51,8 @@ unnecessary time to the test runtime. To avoid this, tests should:
     *   If the test user is logged in using `LoginManagerMixin`, the injected
         `UserContext` has to have the refresh token matching the token passed to
         `FakeGaiaMixin`.
+*   Note that `LoggedInUserMixin` is a compound helper mixin that conveniently
+    packages the mixins mentioned above into an easy-to-use interface.
 
 ## Guest
 
@@ -62,7 +63,7 @@ persisted after the guest session ends.
 To test guest session state, use `GuestSessionMixin` - this will set up
 appropriate guest session flags.
 
-Testing guest user login is more complicated, as guest login required Chrome
+Testing guest user login is more complicated, as guest login requires Chrome
 restart. The test will require two parts:
 *   `PRE_BrowserTest` test that requests login
 *   `BrowserTest` that can test guest session state
@@ -70,4 +71,3 @@ restart. The test will require two parts:
 To properly set up and preserve Chrome flags between sessions runs, use
 `LoginManagerMixin`, and set it up using
 `LoginManagerMixin::set_session_restore_enabled()`
-