[Driver][docs] Document option -mtune as a no-op.

This documents that option -mtune is accepted for compatibility with GCC,
currently it has no effect, and thus does not currently perform any CPU type
specific tuning.

Corresponding discussion on the cfe dev list:
http://lists.llvm.org/pipermail/cfe-dev/2020-April/065169.html

Differential Revision: https://reviews.llvm.org/D78511