[clangd] Config: CompileFlags.Remove

Summary: While here, add documentation to CompileFlags and CompileFlags.Add.

Reviewers: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83705