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...
By changing the other commands to `... where it was when the mark was set` I think we can remove that part and just write "marks the current caret position".

What happens with multiple selections btw?