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

[via cfe-commits]

================
@@ -74,6 +77,40 @@ 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.
 
+Custom functions
+----------------
+
+The options `CustomNormalFunctions` and `CustomAnnexKFunctions` allow the user
+to define custom functions to be checked. The format is the following, without
+newlines:
+
+.. code::
+
+   bugprone-unsafe-functions.CustomNormalFunctions="
+     functionRegex1, replacement1[, reason1]; 
+     functionRegex2, replacement2[, reason2];
+     ...
+   "
+
+The functions are matched using POSIX extended regular expressions.
+*(Note: The regular expressions do not support negative* ``(?!)`` *matches)*
+
+The `reason` is optional and is used to provide additional information about the
+reasoning behind the replacement. The default reason is ``is marked as unsafe``.
----------------
EugeneZelenko wrote:

```suggestion
reasoning behind the replacement. The default reason is `is marked as unsafe`.
```

Option values should be enclosed in single back-ticks. Same below.

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