doc: method and class definition order matters

When defining S7 classes and methods, the order matters whereas typically this is not the case. For example, in `types.R` I have a Todo definition ```r Todo <- S7::new_class( "Todo", properties = list( ... ), ) ``` In `data-frame-methods.R` I have a method defined: ```r method(as.data.frame, Todo) <- function(x, ...) { data.frame( ... ) } ``` This causes an error when using `devtools::load_all()` ``` Error in `load_all()`: ! Failed to load R/data-frame-methods.R Caused by error: ! object 'Todo' not found ``` I believe this is because of the [collation order](https://roxygen2.r-lib.org/articles/collate.html) putting `data-frame-methods.R` before `types.R`. The fix is to either change the file names or to put the method definition after the class definition. Perhaps recommendations can be made in ["Generics and methods"](https://rconsortium.github.io/S7/articles/generics-methods.html)?