[OpenMP] Add dynamic memory function to omp.h and add documentation

This patch adds the `llvm_omp_target_dynamic_shared_alloc` function to
the `omp.h` header file so users can access it by default. Also changed
the name to keep it consistent with the other target allocators. Added
some documentation so users know how to use it. Didn't add the interface
for Fortran since there's no way to test it right now.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D123246