[Docs][llvm-strip] Add help text to llvm-strip rst doc

Summary: Addresses https://bugs.llvm.org/show_bug.cgi?id=42383

Reviewers: jhenderson, alexshap, rupprecht

Reviewed By: jhenderson

Subscribers: wolfgangp, jakehehrlich, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65384

llvm-svn: 368464