environment.py: allow link:run (#29336)

* environment.py: allow link:run

Some users want minimal views, excluding run-type dependencies, since
those type of dependencies are covered by rpaths and the symlinked
libraries in the view aren't used anyways.

With this change, an environment like this:

```
spack:
  specs: ['py-flake8']
  view:
    default:
      root: view
      link: run
```

includes python packages and python, but no link type deps of python.

1 files changed, 20 insertions, 6 deletions

diff --git a/lib/spack/docs/environments.rst b/lib/spack/docs/environments.rst
index ed8e36c9ff..4e4a75b88c 100644
--- a/lib/spack/docs/environments.rst
+++ b/lib/spack/docs/environments.rst
@@ -740,9 +740,10 @@ file snippet we define a view named ``mpis``, rooted at
 version, and compiler name to determine the path for a given
 package. This view selects all packages that depend on MPI, and
 excludes those built with the PGI compiler at version 18.5.
-All the dependencies of each root spec in the environment will be linked
-in the view due to the command ``link: all`` and the files in the view will
-be symlinks to the spack install directories.
+The root specs with their (transitive) link and run type dependencies
+will be put in the view due to the  ``link: all`` option,
+and the files in the view will be symlinks to the spack install
+directories.
 
 .. code-block:: yaml
 
@@ -762,9 +763,22 @@ For more information on using view projections, see the section on
 :ref:`adding_projections_to_views`. The default for the ``select`` and
 ``exclude`` values is to select everything and exclude nothing. The
 default projection is the default view projection (``{}``). The ``link``
-defaults to ``all`` but can also be ``roots`` when only the root specs
-in the environment are desired in the view. The ``link_type`` defaults
-to ``symlink`` but can also take the value of ``hardlink`` or ``copy``.
+attribute allows the following values:
+
+#. ``link: all`` include root specs with their transitive run and link type
+   dependencies (default);
+#. ``link: run`` include root specs with their transitive run type dependencies;
+#. ``link: roots`` include root specs without their dependencies.
+
+The ``link_type`` defaults to ``symlink`` but can also take the value
+of ``hardlink`` or ``copy``.
+
+.. tip::
+
+   The option ``link: run`` can be used to create small environment views for
+   Python packages. Python will be able to import packages *inside* of the view even
+   when the environment is not activated, and linked libraries will be located
+   *outside* of the view thanks to rpaths.
 
 Any number of views may be defined under the ``view`` heading in a
 Spack Environment.