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

doc/Doxyfile.in

@@ -287,7 +287,7 @@ CALLER_GRAPH           = NO
 GRAPHICAL_HIERARCHY    = YES
 DIRECTORY_GRAPH        = YES
 DOT_IMAGE_FORMAT       = svg
-INTERACTIVE_SVG        = YES
+INTERACTIVE_SVG        = NO
 DOT_PATH               =
 DOTFILE_DIRS           =
 MSCFILE_DIRS           =

doc/tutorials/gapi/table_of_content_gapi.markdown

@@ -3,6 +3,20 @@
 In this section you will learn about graph-based image processing and
 how G-API module can be used for that.
 
+- @subpage tutorial_gapi_interactive_face_detection
+
+    *Languages:* C++
+
+    *Compatibility:* \> OpenCV 4.2
+
+    *Author:* Dmitry Matveev
+
+    This tutorial illustrates how to build a hybrid video processing
+    pipeline with G-API where Deep Learning and image processing are
+    combined effectively to maximize the overall throughput. This
+    sample requires Intel® distribution of OpenVINO™ Toolkit version
+    2019R2 or later.
+
 - @subpage tutorial_gapi_anisotropic_segmentation
 
     *Languages:* C++

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