[docs] Further clarify final deprecation procedure

Author: andrewnicols

general/development/policies/deprecation/index.md

@@ -147,7 +147,9 @@ public function foobar(int $old, array $params): array {
 
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-- Keep the existing parameters and return type for both functions and methods.
+- Function and Method signatures **must not change**, that is:
+  - keep any existing parameters; and
+  - keep any existing return type.
 - The deprecation 'since' tag should remain as the version where the initial deprecation happened.
 - Deprecated classes must be completely removed.
 - The content of the PHPDoc should be removed, leaving only the `@deprecated` tag with the notice and, optionally, the replacement information. This includes all `@param`, `@return`, and other tags, as well as the description.