CMakeGraphVizOptions: Minor improvements to docs.

* Add `GRAPHVIZ_GRAPH_TYPE` options.
* Correct inaccurate usage info.

Modules/CMakeGraphVizOptions.cmake

@@ -13,8 +13,8 @@
 # CMake
 # can generate graphviz files, showing the dependencies between the
 # targets in a project and also external libraries which are linked
-# against.  When CMake is run with the --graphviz=foo option, it will
-# produce
+# against.  When CMake is run with the --graphviz=foo.dot option, it will
+# produce:
 #
 # * a foo.dot file showing all dependencies in the project
 # * a foo.dot.<target> file for each target, file showing on which other targets the respective target depends
@@ -29,11 +29,18 @@
 #
 # .. variable:: GRAPHVIZ_GRAPH_TYPE
 #
-#  The graph type
+#  The graph type.
 #
 #  * Mandatory : NO
 #  * Default   : "digraph"
 #
+#  Valid graph types are:
+#
+#  * "graph" : Nodes are joined with lines
+#  * "digraph" : Nodes are joined with arrows showing direction
+#  * "strict graph" : Like "graph" but max one line between each node
+#  * "strict digraph" : Like "graph" but max one line between each node in each direction
+#
 # .. variable:: GRAPHVIZ_GRAPH_NAME
 #
 #  The graph name.