document update threshold for clang-tidy

when "analyze open files" is enabled clang-tidy is executed whenever a file is saved. this causes rather high cpu load (for my current codebase a clang-tidy run takes about 60 second on a reasonably fast machine).

the clangd code model contains a document update threshold to rate-limit clangd. it would be great if something similar could be implemented for clang-tidy. this would reduce the clang-tidy cpu load by rate-limiting it, while still providing direct clang-tidy integration into the editor