doc: add parallelism note to os.cpus()

With the introduction of os.availableParallelism(), users should
no longer rely on os.cpus().length to determine the amount of
available parallelism. This commit adds a note to the os.cpus()
docs.

doc/api/os.md

@@ -138,6 +138,10 @@ The properties included on each object include:
 `nice` values are POSIX-only. On Windows, the `nice` values of all processors
 are always 0.
 
+`os.cpus().length` should not be used to calculate the amount of parallelism
+available to an application. Use
+[`os.availableParallelism()`](#osavailableparallelism) for this purpose.
+
 ## `os.devNull`
 
 <!-- YAML