[clangd] Mention indexing in docs.

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D57392

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@353423 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed