[clang-tools-extra] [clang-tidy][NFC] Add mention of running 'clang-tidy' on changes in Contributing.rst (PR #148547)

[Baranov Victor via cfe-commits]

vbvictor wrote:

> Clang-Tidy pull requests are usually small, so is `-j` really necessary?

I may say why not to use all cores when we can.
If PR only touches 1 file, `clang-tidy-diff` will not load other cores.
If PR touches multiple files, the user can have N times faster linting time, which is always great.

To put into perspective, average lint time of a single `.cpp` check-file is ~20-24 seconds on my machine, so if it is a refactor change in multiple checks at once `-j` can make a big difference.


https://github.com/llvm/llvm-project/pull/148547