doc: non-partitioned async crypto operations

[davisjam]

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.

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, crypto

[davisjam]

Example for crypto.randomBytes():

var nBytes = 10 * 1024*1024; /* 10 MB */
var nLongRequests = 20;
const crypto = require('crypto');

for (var i = 0; i < nLongRequests; i++) {
	crypto.randomBytes(nBytes, (buf) => {
		console.log('Long buf finished');
	});
}

crypto.randomBytes(1, (buf) => {
	console.log('Short buf finished');
});

console.log('begin');

Example for crypto.randomFill():

var nBytes = 10 * 1024*1024; /* 10 MB */
var nLongRequests = 20;
const crypto = require('crypto');

for (var i = 0; i < nLongRequests; i++) {
	var buf = Buffer.alloc(nBytes);
	crypto.randomFill(buf, () => {
		console.log('Long buf finished');
	});
}

var shortBuf = Buffer.alloc(1);
crypto.randomFill(shortBuf, () => {
	console.log('Short buf finished');
});

console.log('begin');

Both of these examples only handle the short request after the threadpool queue empties of the long ones, so "Short buf finished" prints before the final 3 "Long buf finished" messages with UV_THREADPOOL_SIZE=4.

[addaleax]

Landed in 16e87ed

[gibfahn]

If anyone would like to backport to 6.x please feel free (guide)!