[clangd] Mark "Source Hover" as implemented in the docs

Summary: Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>

Reviewers: simark

Reviewed By: simark

Subscribers: klimek, ilya-biryukov, ioeric, MaskRay, cfe-commits

Differential Revision: https://reviews.llvm.org/D45044

llvm-svn: 328792