doc : mention constant-time in crypto doc

Included reference to 'constant time' in
crypto.timingSafeEqual description.

The Node website would score higher on a Google search
and the API would be more discoverable if it used the words
"constant time" in its description.

PR-URL: https://github.com/nodejs/node/pull/16604
Fixes: https://github.com/nodejs/node/issues/16504


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>