Add a chapter about threading layer into the docs (#2848)

- Added a chapter about internal threading primitives into oneDAL documentation
- Improved commenting in the header specifying those primitives APIs (threading.h)

CONTRIBUTING.md

@@ -83,6 +83,12 @@ editorconfig-checker
 
 For your convenience we also added [coding guidelines](http://oneapi-src.github.io/oneDAL/contribution/coding_guide.html) with examples and detailed descriptions of the coding style oneDAL follows. We encourage you to consult them when writing your code.
 
+## Custom Components
+
+### Threading Layer
+
+In the source code of the algorithms, oneDAL does not use threading primitives directly. All the threading primitives used within oneDAL form are called the [threading layer](http://oneapi-src.github.io/oneDAL/contribution/threading.html). Contributors should leverage the primitives from the layer to implement parallel algorithms.
+
 ## Documentation Guidelines
 
 oneDAL uses `Doxygen` for inline comments in public header files that are used to build the API reference and  `reStructuredText` for the Developer Guide. See [oneDAL documentation](https://oneapi-src.github.io/oneDAL/) for reference.

cpp/daal/src/threading/threading.cpp

@@ -103,7 +103,7 @@ DAAL_EXPORT size_t _setNumberOfThreads(const size_t numThreads, void ** globalCo
     return 1;
 }
 
-DAAL_EXPORT void _daal_threader_for(int n, int threads_request, const void * a, daal::functype func)
+DAAL_EXPORT void _daal_threader_for(int n, int reserved, const void * a, daal::functype func)
 {
     if (daal::threader_env()->getNumberOfThreads() > 1)
     {
@@ -160,7 +160,7 @@ DAAL_EXPORT void _daal_threader_for_blocked_size(size_t n, size_t block, const v
     }
 }
 
-DAAL_EXPORT void _daal_threader_for_simple(int n, int threads_request, const void * a, daal::functype func)
+DAAL_EXPORT void _daal_threader_for_simple(int n, int reserved, const void * a, daal::functype func)
 {
     if (daal::threader_env()->getNumberOfThreads() > 1)
     {
@@ -318,7 +318,7 @@ DAAL_PARALLEL_SORT_IMPL(daal::IdxValType<double>, pair_fp64_uint64)
 
 #undef DAAL_PARALLEL_SORT_IMPL
 
-DAAL_EXPORT void _daal_threader_for_blocked(int n, int threads_request, const void * a, daal::functype2 func)
+DAAL_EXPORT void _daal_threader_for_blocked(int n, int reserved, const void * a, daal::functype2 func)
 {
     if (daal::threader_env()->getNumberOfThreads() > 1)
     {