Added more detailed documentation for clangd

Summary:
The text was written mostly by Sam McCall, screenshots are mostly made
by me.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58710

llvm-svn: 354992