[docs] Rewrite/improve the docs for LLVM_NATIVE_TOOL_DIR

Don't include it among the mandatory options; the automatically built
tools via a nested cmake build work fine these days
(in particular, since 93010544 /
https://reviews.llvm.org/D126313).

Clarify the directory path-to-host-bin into something more verbose,
to avoid ambiguity with LLVM_HOST_TRIPLE.

Differential Revision: https://reviews.llvm.org/D142960