Add link_type documentation (#25451)

lib/spack/docs/environments.rst

@@ -732,13 +732,17 @@ Configuring environment views
 The Spack Environment manifest file has a top-level keyword
 ``view``. Each entry under that heading is a view descriptor, headed
 by a name. The view descriptor contains the root of the view, and
-optionally the projections for the view, and ``select`` and
-``exclude`` lists for the view. For example, in the following manifest
+optionally the projections for the view, ``select`` and
+``exclude`` lists for the view and link information via ``link`` and
+``link_type``. For example, in the following manifest
 file snippet we define a view named ``mpis``, rooted at
 ``/path/to/view`` in which all projections use the package name,
 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.
 
 .. code-block:: yaml
 
@@ -751,11 +755,16 @@ excludes those built with the PGI compiler at version 18.5.
          exclude: ['%pgi@18.5']
          projections:
            all: {name}/{version}-{compiler.name}
+         link: all
+         link_type: symlink
 
 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 (``{}``).
+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``.
 
 Any number of views may be defined under the ``view`` heading in a
 Spack Environment.