[clangd] Make -clang-tidy-checks a non-hidden command-line arg

Summary:
This looks like a useful user-facing configuration parameter,
which should be discoverable.

Also fix a small typo in the description.

Reviewers: hokein

Reviewed By: hokein

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D57384

llvm-svn: 352509