clarify naming of intermediate objects in `tune_grid_loop_iter()`

[simonpcouch]

Generally, the naming pattern for objects in tune_grid_loop_iter() follows out_* for objects that will be outputted and iter_* for internal objects. There are several exceptions, though:

https://github.com/tidymodels/tune/blob/f6300b1e2f3c97ef2a4f48e2aa62b8c64cb238ac/R/grid_code_paths.R#L320-L321

https://github.com/tidymodels/tune/blob/f6300b1e2f3c97ef2a4f48e2aa62b8c64cb238ac/R/grid_code_paths.R#L354

https://github.com/tidymodels/tune/blob/f6300b1e2f3c97ef2a4f48e2aa62b8c64cb238ac/R/grid_code_paths.R#L410

https://github.com/tidymodels/tune/blob/f6300b1e2f3c97ef2a4f48e2aa62b8c64cb238ac/R/grid_code_paths.R#L388-L390