[clang-tools-extra] [clang-tidy] Add user-defined functions to `bugprone-unsafe-functions` check (PR #106350)

[via cfe-commits]

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@@ -19,6 +19,8 @@ The check implements the following rules from the CERT C Coding Standard:
 Unsafe functions
 ----------------
 
+The following functions are reported if :ref:`ReportDefaultFunctions<option-ReportDefaultFunctions>` is enabled.
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whisperity wrote:

```suggestion
The following functions are reported if :option:`ReportDefaultFunctions` is enabled.
```

I knew there were examples for this, and now I was able to find some:

https://github.com/llvm/llvm-project/blame/02a334de6690202154ef09456c581618ff290f9a/clang-tools-extra/docs/clang-tidy/checks/misc/unused-parameters.rst#L13

https://github.com/llvm/llvm-project/pull/106350