doc: improved parallel specification

PR-URL: #42679
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

doc/contributing/building-node-with-ninja.md

@@ -22,14 +22,14 @@ ninja: Entering directory `out/Release`
 
 The bottom line will change while building, showing the progress as
 `[finished/total]` build steps. This is useful output that `make` does not
-produce and is one of the benefits of using Ninja. Also, Ninja will likely
-compile much faster than even `make -j4` (or
-`-j<number of processor threads on your machine>`). You can still pass the
-number of processes to run for [Ninja][] using the environment variable `JOBS`.
-This will be the equivalent to the `-j` parameter in the regular `make`:
+produce and is one of the benefits of using Ninja. When using Ninja, builds
+are always run in parallel, based by default on the number of CPUs your
+system has. You can use the `-j` parameter to override this behavior,
+which is equivalent to the `-j` parameter in the regular `make`:
 
 ```bash
-JOBS=12 make
+make -j4 # With this flag, Ninja will limit itself to 4 parallel jobs,
+         # regardless of the number of cores on the current machine.
 ```
 
 ## Producing a debug build