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...
What does `right before the caret` actually mean? From the wording it would be `b` in `ab|cde`, where `|` is the caret, but ST doesn't usually work like that, which is why I'm asking.