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: #16604
Fixes: #16504
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/crypto.md

@@ -1969,6 +1969,7 @@ added: v6.6.0
 - `a` {Buffer | TypedArray | DataView}
 - `b` {Buffer | TypedArray | DataView}
 
+This function is based on a constant-time algorithm.
 Returns true if `a` is equal to `b`, without leaking timing information that
 would allow an attacker to guess one of the values. This is suitable for
 comparing HMAC digests or secret values like authentication cookies or