Altered WeakMap note regarding WeakSet in Firefox

FF Nightly 34's WeakSet allows constructor arguments, even though
WeakMap seemingly doesn't - `new WeakSet([Math]).has(Math)` returns
true, even though `new WeakMap([[Math,1]]).has(Math)` doesn't.
Merge pull request #219 from webbedspace/weakmapnote

Altered WeakMap note regarding WeakSet in Firefox