reword yield_per a bit more
reword yield_per a bit more

I'm still not satisified with this section as it is still
too wordy and dense, but at least let's put a better description
of what yield_per actually is and why one might use it at the top.

Change-Id: I10f4d862d9c499044f5718fca0d27ac106289717

sqlalchemy/sqlalchemy,zzzeek/sqlalchemy