Document that Multilib flags must be actual flags

This is because -print-multi-lib depends on them being flags for correctness.

Fixes a case of this in the arm-android multilib selection logic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@302207 91177308-0d34-0410-b5e6-96231b3b80d8