Clarify what format the `country` is on phone number inputs

Fixes https://github.com/matrix-org/matrix-doc/issues/1982

api/identity/definitions/request_msisdn_validation.yaml

@@ -30,8 +30,8 @@ properties:
   country:
     type: string
     description: |-
-      The two-letter uppercase ISO country code that the number in
+      The two-letter uppercase ISO-3166-1 alpha-2 country code that the
-      ``phone_number`` should be parsed as if it were dialled from.
+      number in ``phone_number`` should be parsed as if it were dialled from.
     example: "GB"
   phone_number:
     type: string

changelogs/client_server/newsfragments/2342.clarification

@@ -0,0 +1 @@
+Clarify the expected phone number format.

specification/client_server_api.rst

@@ -1042,6 +1042,9 @@ wishes to canonicalise the phone number, then it can use the
     "phone": "<The phone number>"
   }
 
+The ``country`` is the two-letter uppercase ISO-3166-1 alpha-2 country code
+that the number in ``phone`` should be parsed as if it were dialled from.
+
 Login
 ~~~~~