add documentation for make_jobs variable (#44838)

* add documentation for make_jobs variable

* apply suggested changes

* Update packaging_guide.rst

add suggestions to the documentation

* Update packaging_guide.rst

fix missing quotes in the documentation

* suggestions to packaging_guide.rst

lib/spack/docs/packaging_guide.rst

@@ -2344,6 +2344,27 @@ you set ``parallel`` to ``False`` at the package level, then each call
 to ``make()`` will be sequential by default, but packagers can call
 ``make(parallel=True)`` to override it.
 
+Note that the `--jobs` option works out of the box for all standard
+build systems. If you are using a non-standard build system instead, you
+can use the variable `make_jobs` to extract the number of jobs specified
+by the `--jobs` option:
+
+.. code-block:: python
+   :emphasize-lines: 7, 11
+   :linenos:
+ 
+   class Xios(Package):
+      ...
+      def install(self, spec, prefix):
+         ...
+         options = [
+            ...
+            '--jobs', str(make_jobs),
+        ]
+        ...
+        make_xios = Executable("./make_xios")
+        make_xios(*options)
+
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Install-level build parallelism
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^