[clangd] Clarify and hide -index flag.
[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

git-svn-id: a34e9779ed74578ad5922b3306b3d80a0c825546@342134 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra