Document sample_size in all signals

[krivard]

Sample size doesn’t have a consistent definition everywhere -- for example, in SafeGraph it's the number of device pools; while in Quidel it's the number of tests. We should add a section to the documentation of each data source explaining the specifics.

[jingjtang]

Quidel uses number of total tests taken in a single day for raw_pct_positive but the sum of test numbers in the 7-day moving average window (after geographical pooling) for smoothed_pct_positive.