Enhancement to module autoload documentation (#10310)

* Note that `none` is the default for lmod autoload

Save a bit of confusion by *explicitly* pointing out that `none` is
the default value for autoload in the lmod module file generator.

* Add a tip re building software externally

Add a tip about using `autoload: all` when building packages outside
of the tree that use artifacts (e.g. libraries, includes) within the
tree.

lib/spack/docs/module_file_support.rst

@@ -618,7 +618,21 @@ activated using ``spack activate``:
 The configuration file above will produce module files that will
 load their direct dependencies if the package installed depends on ``python``.
 The allowed values for the ``autoload`` statement are either ``none``,
-``direct`` or ``all``.
+``direct`` or ``all``.  The default is ``none``.
+
+.. tip::
+  Building external software
+     Setting ``autoload`` to ``direct`` for all packages can be useful
+     when building software outside of a Spack installation that depends on
+     artifacts in that installation.  E.g. (adjust ``lmod`` vs ``tcl``
+     as appropriate):
+
+  .. code-block:: yaml
+
+     modules:
+       lmod:
+         all:
+           autoload: 'direct'
 
 .. note::
   TCL prerequisites