doc: crypto.randomBytes does not block when async

It may not return random bytes right away, but when called
asynchronously it will not block.

PR-URL: nodejs/node#14993
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/crypto.md

@@ -1717,7 +1717,8 @@ console.log(
   `${buf.length} bytes of random data: ${buf.toString('hex')}`);
 ```
 
-The `crypto.randomBytes()` method will block until there is sufficient entropy.
+The `crypto.randomBytes()` method will not complete until there is
+sufficient entropy available.
 This should normally never take longer than a few milliseconds. The only time
 when generating the random bytes may conceivably block for a longer period of
 time is right after boot, when the whole system is still low on entropy.