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...
Okay, that makes sense to me.

Things get funny!. It turns out you can actually have multiple marks in the same file - you need only run `set_mark` when using multiple carets. Related to this, `swap_with_mark` marks all your current carets' positions, drops those carets, and sets new ones at the previously marked positions (this generalizes to whenever any case is just one caret). If no caret(s) have been set, `swap_with_mark` actually just runs `set_mark`.

When using multiple carets, the behaviour of  `select_to_caret` is a bit tricky, but I think I have discovered it completely. This applies as well to when we have only one mark or only one caret. I will use the word 'extend' (in all Sublime fashion :stuck_out_tongue_winking_eye:) to mean "select text" as per "Selects the text between the current position of the caret and the position where it was when the mark was set". So, the thing goes like this:
* Each caret is expanded to each mark, in respective order or occurrence (1st caret to 1st mark, etc.). The caret is preserved at its position if it occurred before the mark it is expanded to, or moved to the marked position where its expansion ends, if it occurs after the mark it is expanded to.
* If any number of selections overlap, they are joined (as in "union") and, of all the carets corresponding to each one of the joined selections, only the one occurring first in the file is preserved.
* If one specifies a less or equal number of carets than marks, only the first some marks are expanded. However, If more carets are specified, only the first "#-of-marks" carets are expanded, respectively in order, and each of those selections might contain other, extra carets (this will definitely happen if the extra carets lie __before__ the last mark), which are dumped.
Those not overridden (only the ones that lie __after__ the last mark) are just left where they are after all expansions are made, without triggering a selection from their position (because there is no mark to which they could be expanded).

Finally, `delete_to_mark` simply works as if running `select_to_mark` and then pressing `Del`. Obviously, this covers all cases of multiple carets.

How shall we go about writing this in a clearer way? Would describing the behaviour in a step-by-step manner, as above, be acceptable here?