message: Need -DBUILD_DOCS CMake flag to build docs
  Must have that flag. Otherwise following "doxygen overview" tutorial won't work.
diff: @@ -32,11 +32,11 @@ Generate documentation {#tutorial_documentation_generate}
  - Create build directory near the sources folder(s) and go into it
  - Run cmake (assuming you put sources to _opencv_ folder):
  @code{.sh}
  - cmake ../opencv
  + cmake -DBUILD_DOCS=ON ../opencv
  @endcode
  Or if you get contrib sources too:
  @code{.sh}
  - cmake -DOPENCV_EXTRA_MODULES_PATH=../opencv_contrib/modules ../opencv
  + cmake -DBUILD_DOCS=ON -DOPENCV_EXTRA_MODULES_PATH=../opencv_contrib/modules ../opencv
  @endcode
  - Run make:
  @code{.sh}
  