Come to think of it, there's no point touching `document_rename_file()` because it's not useful being used alone, and it shouldn't be used alone since you would need to have everyone that listens to "document-save" to be updated. It probably even needs to be dropped from the API.
We better just create another function named `document_rename_file_and_save()` (i.e. `gboolean document_rename_file_and_save(GeanyDocument *doc, const gchar *new_filename);`) for anyone that would want to use it. An example for this usage is an in-place renaming of a document in Documents tab.