Document some 'mark'-related commands. (+ fix small typo)

I can't really figure out what 'yank' does. It seems to paste the text deleted when running 'delete_to_mark' but that doesn't seem to be coherent with its name...
Re caret of the selections: So, in fact the resulting selections will always have the caret at the beginning (`.begin()`)? If so, we should write that instead.

I think this information can be split reasonably across the four commands.