Remove enum for `POST /login` `type` definition (#1776)

Since the enum is not exhaustive, improve the description of the property instead.

changelogs/client_server/newsfragments/1776.clarification

@@ -0,0 +1 @@
+Clarify that the `type` of the `POST /login` request must be one of the types returned by the `GET /login` response.

data/api/client-server/login.yaml

@@ -109,10 +109,12 @@ paths:
               properties:
                 type:
                   type: string
-                  enum:
-                    - m.login.password
-                    - m.login.token
-                  description: The login type being used.
+                  description: |-
+                    The login type being used.
+
+                    This must be a type returned in one of the flows of the
+                    response of the [`GET /login`](/client-server-api/#get_matrixclientv3login)
+                    endpoint, like `m.login.password` or `m.login.token`.
                 identifier:
                   $ref: definitions/user_identifier.yaml
                 user: