doc: add information about Assert behavior and maintenance

Assert is now locked. Userland alternatives should be used. Assert is
for testing Node.js itself.

Document potentially surprising use of enumerable properties only in
deep equality assertions.

Ref: https://github.com/nodejs/node/pull/3124
Ref: https://github.com/nodejs/node/issues/3122

PR-URL: https://github.com/nodejs/node/pull/3330


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Rod Vagg <rod@vagg.org>