[clangd] Fix the stale documentation about background indexing.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66937

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@372825 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed