[clangd] Add docs for configuration knobs in clangd

Summary:
This is a first step in documenting different configuration knobs we
have in clangd.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66740

llvm-svn: 369917