Documentation improvement: Define simple operation

[alydh-annotell]

For the option skip_confirm_for_simple_edits there is no definition what a simple edit/operation is.

Looking at lua/oil/mutator/preview.lua is_simple_edit it seems like everything but the follow list is considered simple.

• delete
• copy/move across adapters
• multiple moves or copies at the same time
• more than 5 creates

Could this be added to the docs somewhere?

[stevearc]

Added an explanation to the help docs