[clangd] improve help message for limit-results

Summary: Make it clear that the default is 100.

Patch by Brennan Vincent(@umanwizard)!

Reviewers: #clang-tools-extra, kadircet

Reviewed By: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61865

llvm-svn: 361388