Add better 'wdir' documentation

[gradyking]

Hi!

I think the documentation for 'wdir', or wind direction, is a bit unclear, and it was challenging to find out exactly what it means.

I figured out that 0 degrees means north, 90 degrees means east, 180 south, and 270 west, but I only figured that out by comparing two API calls, one with unconverted 'wdir' in degrees, and the other converted to cardinal directions with stationData.convert({'wdir': units.direction}).

I think updating this documentation with a better description would help in the future.

Thanks!