message: Added documentation for `buildKit` expansion variable
diff: @@ -249,6 +249,9 @@ using ``${variable}`` syntax. The following built-in variables are expanded:
  ``${buildType}``
  The current CMake build type, eg. ``Debug``, ``Release``, ``MinSizeRel``
  +``${buildKit}``
  + The current CMake kit name, eg. ``GCC 7.3.0``
  +
  ``${generator}``
  The name of the CMake generator, eg. ``Ninja``
  