[clang-tools-extra] [clang-tidy][NFC] Update contribution guide for test language standards (PR #184769)

[Baranov Victor via cfe-commits]

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@@ -662,6 +662,28 @@ Here's an example:
    // CHECK-FIXES-USING-B-NOT: using a::B;$
    // CHECK-FIXES-NOT: using a::C;$
 
+Specifying the Language Standard
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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vbvictor wrote:

Previously docs used `------------` instead of `^^^^^^`

https://github.com/llvm/llvm-project/pull/184769