Document some of the admin APIs (#2143)

I haven't (yet) documented all of the user-list APIs introduced in
https://github.com/matrix-org/synapse/pull/1784 because the API shape seems
very odd, given the functionality.

docs/admin_api/user_admin_api.rst

+Query Account
+=============
+
+This API returns information about a specific user account.
+
+The api is::
+
+    GET /_matrix/client/r0/admin/whois/<user_id>
+
+including an ``access_token`` of a server admin.
+
+It returns a JSON body like the following:
+
+.. code:: json
+
+    {
+        "user_id": "<user_id>",
+        "devices": {
+            "": {
+                "sessions": [
+                    {
+                        "connections": [
+                            {
+                                "ip": "1.2.3.4",
+                                "last_seen": 1417222374433,
+                                "user_agent": "Mozilla/5.0 ..."
+                            },
+                            {
+                                "ip": "1.2.3.10",
+                                "last_seen": 1417222374500,
+                                "user_agent": "Dalvik/2.1.0 ..."
+                            }
+                        ]
+                    }
+                ]
+            }
+        }
+    }
+
+``last_seen`` is measured in milliseconds since the Unix epoch.
+
+Deactivate Account
+==================
+
+This API deactivates an account. It removes active access tokens, resets the
+password, and deletes third-party IDs (to prevent the user requesting a
+password reset).
+
+The api is::
+
+    POST /_matrix/client/r0/admin/deactivate/<user_id>
+
+including an ``access_token`` of a server admin, and an empty request body.
+
+
+Reset password
+==============
+
+Changes the password of another user.
+
+The api is::
+
+    POST /_matrix/client/r0/admin/reset_password/<user_id>
+
+with a body of:
+
+.. code:: json
+
+   {
+       "new_password": "<secret>"
+   }
+
+including an ``access_token`` of a server admin.