[clang-tools-extra] [clang-tidy] Contributing.rst update snippet and docs (PR #129209)

[Carlos Galvez via cfe-commits]

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@@ -203,6 +202,10 @@ for more information) that will find the pattern in the AST that we want to
 inspect. The results of the matching are passed to the ``check`` method, which
 can further inspect them and report diagnostics.
 
+By default, the new check applies only to C++ code. If it should apply under 
+different language options, be sure to update the ``isLanguageVersionSupported``
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carlosgalvezp wrote:

Would it be better to document that one can specify the language when running `add_new_check.py --language`? Possibly earlier in this document when it describes what this script does.


https://github.com/llvm/llvm-project/pull/129209