Merge pull request #16039 from dmatveev:dm/gapi_tutorial_interactive_face_detection

* G-API-NG/Docs: Added a tutorial page on interactive face detection sample

- Introduced a "--ser" option to run the pipeline serially for
  benchmarking purposes
- Reorganized sample code to better fit the documentation;
- Fixed a couple of issues (mainly typos) in the public headers

* G-API-NG/Docs: Reflected meta-less compilation in new G-API tutorial

* G-API-NG/Docs: Addressed review comments on Face Analytics Pipeline example

modules/gapi/include/opencv2/gapi/streaming/source.hpp

@@ -24,12 +24,13 @@ namespace wip {
  * Implement this interface if you want customize the way how data is
  * streaming into GStreamingCompiled.
  *
- * Objects implementing this interface can be passes to
- * GStreamingCompiled via setSource()/cv::gin(). Regular compiled
- * graphs (GCompiled) don't support input objects of this type.
+ * Objects implementing this interface can be passed to
+ * GStreamingCompiled using setSource() with cv::gin(). Regular
+ * compiled graphs (GCompiled) don't support input objects of this
+ * type.
  *
  * Default cv::VideoCapture-based implementation is available, see
- * cv::gapi::GCaptureSource.
+ * cv::gapi::wip::GCaptureSource.
  *
  * @note stream sources are passed to G-API via shared pointers, so
  *  please use ptr() when passing a IStreamSource implementation to