[LangRef] Clarify Varargs forwarding for musttail calls.

This clarification was suggested by @efriedma in D41335, which uses this
behavior to inline musttail calls with varargs.

Reviewers: hfinkel, efriedma, rnk

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D41861

llvm-svn: 322786