[clang-tools-extra] Fix #41439: Update the documentation with the correct information. (PR #69377)

[Piotr Zegar via cfe-commits]

================
@@ -10,7 +10,12 @@ Guide:
 
 https://google.github.io/styleguide/cppguide.html#Function_Declarations_and_Definitions
 
-All parameters should be named, with identical names in the declaration and
-implementation.
+A parameter name may be omitted only if the parameter is not used in the
+function's definition.
+
+.. code-block:: c++
----------------
PiotrZSL wrote:

format this as:

```
.. code-block:: c++

  int doingSomething(int a, int b, int) {  // Ok: the third paramet is not used
      return a + b;
  }
```

https://github.com/llvm/llvm-project/pull/69377