[clangd] Clarify and hide -index flag.

Summary:
The wording implies global index support, which is confusing.
As most users shouldn't care about this flag, also make it hidden to avoid
further confusion.

Reviewers: sammccall, ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D51977

llvm-svn: 342134