doc: document asserts Weak(Map|Set) behavior

Right now it is not documentated that WeakMap entries are not
compared. This might result in some confusion. This adds a note
about the behavior in `assert.deepStrictEqual`. This documentation
is also references in `util.isDeepStrictEqual`, so we do not have
to document it again for that function as the underlying algorithm
is the same.

Backport-PR-URL: https://github.com/nodejs/node/pull/19230
PR-URL: https://github.com/nodejs/node/pull/18248
Fixes: https://github.com/nodejs/node/issues/18228
Refs: https://github.com/nodejs/node/pull/18228


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>