Help: Update MSVC_VERSION documentation

Add the corresponding toolset name for each version.  While at it,
link to the general compiler version variable.

Help/variable/MSVC_VERSION.rst

@@ -5,14 +5,15 @@ The version of Microsoft Visual C/C++ being used if any.
 
 Known version numbers are::
 
-  1200 = VS  6.0
-  1300 = VS  7.0
-  1310 = VS  7.1
-  1400 = VS  8.0
-  1500 = VS  9.0
-  1600 = VS 10.0
-  1700 = VS 11.0
-  1800 = VS 12.0
-  1900 = VS 14.0
-  1910 = VS 15.0
-  1911 = VS 15.0
+  1200      = VS  6.0
+  1300      = VS  7.0
+  1310      = VS  7.1
+  1400      = VS  8.0 (v80 toolset)
+  1500      = VS  9.0 (v90 toolset)
+  1600      = VS 10.0 (v100 toolset)
+  1700      = VS 11.0 (v110 toolset)
+  1800      = VS 12.0 (v120 toolset)
+  1900      = VS 14.0 (v140 toolset)
+  1910-1919 = VS 15.0 (v141 toolset)
+
+See also the  :variable:`CMAKE_<LANG>_COMPILER_VERSION` variable.