Update docs regarding frozen string

xref #47424.

changelogs/unreleased/frozen-string-docs.yml

+---
+title: Update docs regarding frozen string
+merge_request:
+author: gfyoung
+type: other

doc/development/performance.md

@@ -364,8 +364,7 @@ Depending on the size of the String and how frequently it would be allocated
 there's no guarantee it will.
 
 Strings will be frozen by default in Ruby 3.0. To prepare our code base for
-this eventuality, it's a good practice to add the following header to all
-Ruby files:
+this eventuality, we will be adding the following header to all Ruby files:
 
 ```ruby
 # frozen_string_literal: true
@@ -379,6 +378,9 @@ test = +"hello"
 test += " world"
 ```
 
+When adding new Ruby files, please check that you can add the above header,
+as omitting it may lead to style check failures.
+
 ## Anti-Patterns
 
 This is a collection of [anti-patterns][anti-pattern] that should be avoided