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

doc/api/os.md

@@ -142,6 +142,9 @@ The properties included on each object include:
 `nice` values are POSIX-only. On Windows, the `nice` values of all processors
 are always 0.
 
+On Linux and Android when `/proc` filesystem is not accessible, this function 
+may return an empty list.
+
 `os.cpus().length` should not be used to calculate the amount of parallelism
 available to an application. Use
 [`os.availableParallelism()`](#osavailableparallelism) for this purpose.