Fix Crab Shaft cross room space jump strat, add notes

[blkerby]

I checked this out thoroughly with TAStudio and am confident there's no way to make it onto the floating platform with a 12-tile runway Space Jump; the best candidate is extra run speed $3.1 (as the jump height drops off at $3.2), and it comes up about half a tile short still, even with last-frame Space Jump, lowest possible position in the doorway, and latest possible turnaround. Based on the note, the 18.4375 runway length is what was intended here; I must have just put in the wrong number.

I think it would be useful at some point to add a node below the floating platform, as there are many ways of getting there without necessarily being able to climb onto the floating platform; and with a flash suit, from there you could spark up either through the door or onto the floating platform.