PHP 8.4 deprecates implicitly nullable parameters, i.e. typed parameters with a `null` default value, which are not explicitly declared as nullable.
As the minimums supported PHP version of this code base is PHP 5.6, adding the nullability operator to the type declaration is not an option at this time.
In this case, however, the parameter is found in the declaration of a `private` method, so removing the type declaration in favour of in-function type checking solves the deprecation without breaking BC (as `private`).
Includes updating the documentation to match (where relevant, i.e. only existing documentation has been touched).
Ref: https://wiki.php.net/rfc/deprecate-implicitly-nullable-types