docs: add note about compilers with modules (#9520)

compilers.yaml can track a module that is needed for a compiler, but
Spack does not fill this in automatically. This adds a note to the
documentation informing the user how to do this.

lib/spack/docs/getting_started.rst

@@ -202,6 +202,13 @@ installed, but you know that new compilers have been added to your
 This loads the environment module for gcc-4.9.0 to add it to
 ``PATH``, and then it adds the compiler to Spack.
 
+.. note::
+
+   By default, spack does not fill in the ``modules:`` field in the
+   ``compilers.yaml`` file.  If you are using a compiler from a
+   module, then you should add this field manually.
+   See the section on :ref:`compilers-requiring-modules`.
+
 .. _cmd-spack-compiler-info:
 
 ^^^^^^^^^^^^^^^^^^^^^^^
@@ -320,6 +327,7 @@ by adding the following to your ``packages.yaml`` file:
      all:
        compiler: [gcc@4.9.3]
 
+.. _compilers-requiring-modules:
 
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Compilers Requiring Modules