[css-paint-api] Add in global scope selection rules, i.e. not reusing…

… the same GS more than 1000 times in a row.

css-paint-api/Overview.bs

@@ -565,14 +565,8 @@ with), given |snappedConcreteObjectSize| it <em>must</em> run the following step
  </div>
 
  12. Let |workletGlobalScope| be a {{PaintWorkletGlobalScope}} from the list of <a>worklet's
- WorkletGlobalScopes</a> from the paint {{Worklet}}.
-
- The user agent <em>must</em> have, and select from at least two {{PaintWorkletGlobalScope}}s
- in the <a>worklet's WorkletGlobalScopes</a> <a>list</a>, unless the user agent is under
- memory constraints.
-
- Note: This is to ensure that authors do not rely on being able to store state on the global
- object or non-regeneratable state on the class.
+ WorkletGlobalScopes</a> from the paint {{Worklet}}, following the rules defined in
+ [[#global-scope-selection]].
 
  The user agent <em>may</em> also <a>create a WorkletGlobalScope</a> at this time, given the
  paint {{Worklet}}.
@@ -716,6 +710,23 @@ Note: The contents of the resulting image are not designed to be accessible. Aut
  any useful information through the standard accessibility APIs.
 </div>
 
+Global Scope Selection {#global-scope-selection}
+------------------------------------------------
+
+When the user agent needs to select a {{PaintWorkletGlobalScope}} from the paint <a>worklet's
+WorkletGlobalScopes</a> <a>list</a> it <em>must</em>:
+
+ - Select from at <em>least</em> two {{PaintWorkletGlobalScope}}s, unless the user agent is under
+ memory constraints.
+
+ - <em>Not</em> re-use the same {{PaintWorkletGlobalScope}} more than 1000 times in a row.
+
+ Note: The 1000 limit was picked as a high upper bound, this limit may improve (downwards)
+ over time.
+
+Note: These rules exist to ensure that authors do not rely on being able to store state on the
+ global object or non-regeneratable state on the class. See [[worklets-1#code-idempotency]].
+
 Examples {#examples}
 ====================