Adds ability to add error return documetation to the binder and class reference.
Usage example:
```C++
void MyClass::_bind_method() {
[..]
BIND_METHOD_ERR_RETURN_DOC("load", ERR_FILE_CANT_OPEN, ERR_FILE_UNRECOGNIZED);
}
```
One function of ConfigFile was changed as example.
96f8254b24