Specify our usage of ABNF for grammar (#1582)

* Specify our usage of ABNF for grammar

* Create 1582.clarification

* Update meta/documentation_style.rst

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

---------

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

changelogs/internal/newsfragments/1582.clarification

@@ -0,0 +1 @@
+Clarify usage of ABNF for grammar in the documentation style guide.

meta/documentation_style.rst

@@ -185,3 +185,10 @@ The subheadings which are always deemed as client-affecting are:
 * Redactions
 
 When a new subheading is added, it must be referenced and ordered in this document.
+
+Describing grammar
+~~~~~~~~~~~~~~~~~~
+
+Use  `RFC5234-style ABNF <https://datatracker.ietf.org/doc/html/rfc5234>`_ when describing
+the grammar for something in the spec, such as user IDs or server names. Use lowercase
+and underscore-deliminated element names (`user_id`, not `UserID` or `user-id`).