[clangd] Mark "Document Symbols" as implemented in the docs

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

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

Differential Revision: https://reviews.llvm.org/D48996

llvm-svn: 336550