Tweak description of restrictions on generated functions (#21357)

doc/src/manual/metaprogramming.md

@@ -876,8 +876,10 @@ When defining generated functions, there are four main differences to ordinary f
    function.
 3. Instead of calculating something or performing some action, you return a *quoted expression* which,
    when evaluated, does what you want.
-4. Generated functions must not have any side-effects or examine any non-constant global state (including,
-   for example, IO, locks, or non-local dictionaries). In other words, they must be completely pure.
+4. Generated functions must not *mutate* or *observe* any non-constant global state (including,
+   for example, IO, locks, non-local dictionaries, or using `method_exists`).
+   This means they can only read global constants, and cannot have any side effects.
+   In other words, they must be completely pure.
    Due to an implementation limitation, this also means that they currently cannot define a closure
    or untyped generator.