Update documentation for snow events parameter for Townsend snow loss model

[aturabi]

I reached out to Tim Townsend and got this guidance for how to calculate the "snow events" parameter:

• An event has a 1" threshold.
• Since the std datalogging method is to record snowfall once per day (usually at midnight), the max events per day is one.

See this discussion for more details: https://groups.google.com/u/1/g/pvlib-python/c/j4FEErBul3U

Updating the documentation to reflect this guidance.