[DOC beta] Add documentation to PromiseProxyMixin.
[Bugfix beta] Attribute bindings with a falsy value should always remove the attribute.
Merge pull request #4044 from jmonma/master

Attribute bindings with a falsy value should always remove the attribute...
Merge pull request #4038 from rjackson/document_promise_proxy

[DOC beta] Add documentation to PromiseProxyMixin.