[clangd] Move inserted include from detail -> documentation.

Summary: Many clients try to display all the detail inline, with poor results.

See https://github.com/clangd/clangd/issues/284

Reviewers: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79106