[SYCL][Docs] Add new rule for naming C++ identifiers in SYCL project (#5843)

This adds a new rule of naming C++ identifiers in `llvm/sycl` files. It aims to exclude possibility to fail SYCL app compilation by defining macros before including `CL/sycl.hpp`.

Author: HabKaffee

sycl/doc/developer/ContributeToDPCPP.md

@@ -62,6 +62,9 @@ end-to-end or SYCL-CTS tests.
 - Add a helpful comment describing what the test does at the beginning and
   other comments throughout the test as necessary.
 
+- All identifiers used in `llvm/sycl` headers files must contain at
+  least one lowercase letter due to avoid conflicts with user-defined macros.
+
 - Try to follow descriptive naming convention for variables, functions as
   much as possible. Please refer to
   [LLVM naming convention](https://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly)