Clarify our stance on en_GB vs en_US

changelogs/internal/newsfragments/2366.clarification

@@ -0,0 +1,2 @@
+Clarify use of UK vs US English in documentation style doc.
+

meta/documentation_style.rst

@@ -47,7 +47,20 @@ General
 * Try to write clearly and unambiguously. Remember that many readers will not
   have English as their first language.
 
-* Prefer British English (colour, -ise) to American English.
+* The Matrix spec uses British English rather than American English. For example, the
+  words "colour" and "authorise" use British spellings. The
+  `join_authorised_via_users_server` property in `m.room.member` events is
+  spelt accordingly.
+
+  * This extends to terms defined in other specifications: for example, in the
+    context of OAuth 2.0, we refer to an "authori**s**ation code grant", even though
+    RFC6749 uses the spelling "authori**z**ation".
+
+    Of course, identifiers used within the protocol itself must use the
+    spellings defined by the protocol. So, for example, the
+    `authorization_endpoint` property in the [OAuth 2.0
+    metadata](https://spec.matrix.org/v1.18/client-server-api/#get_matrixclientv1auth_metadata)
+    uses the same spelling as defined in RFC8414.
 
 * The word "homeserver" is spelt thus (rather than "home server", "Homeserver",
   or (argh) "Home Server"). However, an identity server is two words.