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>