[llvm][cmake] Follow up to D117973

1. Slightly document the "mark advanced" variable used to control the
   installed CMake package dir.

   I would document it more, but I am considering in the future adding
   pkg-config support in this manner, after which `_PACKGE_DIR` is
   probably better called `_CMAKE_PACKGE_DIR` or similar.

2. Convey the custom path to the legacy `llvm-config` binary.

Reviewed By: sebastian-ne

Differential Revision: https://reviews.llvm.org/D130539