Help: Add explicit <PackageName>_ROOT variable documentation

Add documentation for both the CMake variable and environment variable
of this name pattern.  Update references to these names to link to their
documents.  Clarify the pattern used to construct their names.

Help/variable/PackageName_ROOT.rst

@@ -0,0 +1,14 @@
+<PackageName>_ROOT
+------------------
+
+Calls to :command:`find_package(<PackageName>)` will search in prefixes
+specified by the ``<PackageName>_ROOT`` CMake variable, where
+``<PackageName>`` is the name given to the ``find_package`` call
+and ``_ROOT`` is literal.  For example, ``find_package(Foo)`` will search
+prefixes specified in the ``Foo_ROOT`` CMake variable (if set).
+See policy :policy:`CMP0074`.
+
+This variable may hold a single prefix or a
+:ref:`;-list <CMake Language Lists>` of multiple prefixes.
+
+See also the :envvar:`<PackageName>_ROOT` environment variable.