[doc] builtin should document the behaviour of `weaken` and `unweaken` on non-references

[tobyink]

Where builtin

Description builtin should document what weaken and unweaken do when called on non-references.

(Pretty sure the answer is that they're a no-op.)

[tonycoz]

They both croak for defined non-references.

[jkeenan]

They both croak for defined non-references.

Please review https://github.com/Perl/perl5/pull/19965 for tests and documentation.