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

[via cfe-commits]

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@@ -74,10 +75,50 @@ Both macros have to be defined to suggest replacement functions from *Annex K.*
 ``__STDC_WANT_LIB_EXT1__`` must be defined to ``1`` by the user **before**
 including any system headers.
 
+.. _CustomFunctions:
+
+Custom functions
+----------------
+
+The option `CustomFunctions` allows the user to define custom functions to be
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whisperity wrote:

```suggestion
The option :option:`CustomFunctions` allows the user to define custom functions to be
```

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