[clangd] Address FIXME and fix comment

* Address a FIXME by warning the user that both -run-synchronously and -j X are
  passed.
* Fix a comment to suppress clang-tidy warning by passing the correct argument
  name.

Reviewers: ioeric

Subscribers: ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43671

llvm-svn: 326051