Update docs for reset-baseline() in vertical_rhythm partial

[JohnAlbin]

Over in #780 the decision was made to deprecate the reset-baseline() partial in the master branch that will be Compass 0.13 (which I did in #790).

But we still need to better document that mixin in the stable 0.12 branch.

Here's a better description of the mixin with a mild warning about it not working in all circumstances.

[ry5n]

@JohnAlbin +1, although I'd make it clear why it won't work and when: “does not work on elements whose font-size is different from $base-font-size”

[scottdavis]

Go ahead and fix the typo and just make it @include adjust-leading-to(1, $base-font-size); and put a @warn in there and ill merge this in.

[chriseppstein]

I took @ry5n's feedback into account -- this is on stable now as 3f96108 and 1aca960