[SYCL] Need to fix warnings related to Doxygen documentation build

A bunch of warnings for Doxygen code documentation is generated during build (see in Actions, e.g.
https://github.com/intel/llvm/actions/runs/1032341110)

Example of failures:
```
clang/lib/Sema/SemaLookup.cpp:765:
warning: parameter 'IsVariadic' is not documented

# parameter types were documented instead of parameters, can be easily fixed
sycl/include/CL/sycl/detail/pi.h:1535:
warning: parameter 'properties' and parameter ‘size’ of piextUSMDeviceAlloc are not documented
```