[docs] Note about how to handle 'llvm-mt: error: no libxml2'

See https://github.com/llvm/llvm-project/issues/55817 and
https://discourse.llvm.org/t/cannot-cmake-self-hosted-clang-on-windows-for-lack-of-libxml2/58793

Not sure where is the best place to put this, but hopefully this will be
found by those searching for the error message.

Differential revision: https://reviews.llvm.org/D129770