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...
Same thing here:

> Deletes the text between the current position of the caret and the position where it was when the mark was set.

Does it look all right?