doc: non-partitioned async crypto operations

Neither crypto.randomBytes nor crypto.randomFill
partitions the work submitted to the threadpool.

This change was suggested during the discussion of #17054.
See also #17154.

PR-URL: #17250
Refs: #17154
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/crypto.md

@@ -1826,6 +1826,11 @@ Note that this API uses libuv's threadpool, which can have surprising and
 negative performance implications for some applications, see the
 [`UV_THREADPOOL_SIZE`][] documentation for more information.
 
+*Note*: The asynchronous version of `crypto.randomBytes()` is carried out
+in a single threadpool request. To minimize threadpool task length variation,
+partition large `randomBytes` requests when doing so as part of fulfilling a
+client request.
+
 ### crypto.randomFillSync(buffer[, offset][, size])
 <!-- YAML
 added: v7.10.0
@@ -1933,6 +1938,11 @@ Note that this API uses libuv's threadpool, which can have surprising and
 negative performance implications for some applications, see the
 [`UV_THREADPOOL_SIZE`][] documentation for more information.
 
+*Note*: The asynchronous version of `crypto.randomFill()` is carried out
+in a single threadpool request. To minimize threadpool task length variation,
+partition large `randomFill` requests when doing so as part of fulfilling a
+client request.
+
 ### crypto.setEngine(engine[, flags])
 <!-- YAML
 added: v0.11.11