[clang-tools-extra] [clang-tidy] Document that check_clang_tidy.py needs unix diff (PR #214681)

[Baranov Victor via cfe-commits]

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@@ -37,6 +37,9 @@
 
 Notes
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+  Failed-fix dumps use the Unix ``diff`` command (not ``git diff``); ensure
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vbvictor wrote:

Yes I agree, as stated above:
> we ideally need to add some sanity-check for it before all tests run, not just comment here

I bet almost nobody look at check_clang_tidy.py so this comment is effectively a no-op

https://github.com/llvm/llvm-project/pull/214681