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

[via cfe-commits]

================
@@ -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
+checked. The format is the following, without newlines:
+
+.. code::
+
+   bugprone-unsafe-functions.CustomFunctions="
+     functionRegex1[, replacement1[, reason1]]; 
+     functionRegex2[, replacement2[, reason2]];
+     ...
+   "
+
+The functions are matched using POSIX extended regular expressions.
+*(Note: The regular expressions do not support negative* ``(?!)`` *matches)*
----------------
whisperity wrote:

```suggestion
*(Note: The regular expressions do not support negative* ``(?!)`` *matches.)*
```

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