Clarify formats of string types for the `POST /_matrix/client/v3/login` endpoint (#1980)

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

changelogs/client_server/newsfragments/1980.clarification

@@ -0,0 +1 @@
+Clarify formats of string types.

data/api/client-server/login.yaml

@@ -172,6 +172,8 @@ paths:
                 properties:
                   user_id:
                     type: string
+                    format: mx-user-id
+                    pattern: "^@"
                     description: The fully-qualified Matrix ID for the account.
                   access_token:
                     type: string
@@ -197,6 +199,7 @@ paths:
                     x-addedInMatrixVersion: "1.3"
                   home_server:
                     type: string
+                    format: mx-server-name
                     deprecated: true
                     description: |-
                       The server_name of the homeserver on which the account has