[libc++][docs] Update documentation to reflect libc++'s compiler support policy

In https://lists.llvm.org/pipermail/llvm-dev/2021-March/148881.html, we
discussed updating the compiler support policy for libc++ to match more
closely what we do actually support.

This commit enshrines that policy decision in libc++'s documentation.

Differential Revision: https://reviews.llvm.org/D105563