[clang-tools-extra] readability-identifier-naming should add universal default options (PR #171686)

[Baranov Victor via cfe-commits]

================
@@ -44,6 +44,35 @@ settings. The options and their corresponding values are:
  - ``LowerCase`` - example: ``int i_Variable``
  - ``CamelCase`` - example: ``int IVariable``
 
+The check only works on kinds of identifiers which have been configured,
+so an empty config effectively disables it.
+The "default" option can be used to enable all kinds of identifiers,
+then optionally override specific kinds which are desired with a different case.
----------------
vbvictor wrote:

```suggestion
The check only enforce style on kinds of identifiers which have been
configured, so an empty config effectively disables it.
The :option:`DefaultCase` option can be used to enforce style on all kinds of
identifiers, then optionally override specific kinds which are desired with a
different case.
```

https://github.com/llvm/llvm-project/pull/171686