devel/llvm-{cheri,morello}: explain --start/end-no-unused-arguments use

I'd misunderstood the case where --start/end-no-unused-arguments were
needed in my previous commit. Add a comment with the correct
explanation which is not future proofing, but instead that some
invocations of the compiler (e.g., clang -v) don't end up consuming
the -Xclang arguments.

Reported by: jrtc27
Fixes: 1cf80317e3ec devel/llvm-{cheri,morello}: future-proof ABI arguments