doc: add a note about os.cpus() returning an empty list

It is not obvious that in some cases cpus() returns an empty list and
this has caused a bug before:
isaacs/promise-call-limit#11

PR-URL: nodejs#47363
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/os.md

@@ -76,6 +76,8 @@ added: v0.3.3
 * Returns: {Object\[]}
 
 Returns an array of objects containing information about each logical CPU core.
+The array will be empty if no CPU information is available, such as if the
+`/proc` file system is unavailable.
 
 The properties included on each object include: