docs: minor updates

[henryiii]

This is a followup to #2, with one more possible change. This is a bit of extra typing, but is much clearer; I could see the following to be rather confusing if this is named counts instead of effective_counts, since it's not always the true number of counts, but an effective count calculation. For a user filling a weighed sum or weighed mean histogram, the fact that ".counts" is neither the number of times filled nor the weighted number of times filled is likely to be quite a surprise. While if it was called "effective_counts", they would be more likely to suspect that it's a special computed quantity and not just the raw or weighted counts.

Followup to #2, CC @HDembinski and @jpivarski.

[jpivarski]

I'm going to bring Uproot in line with histogram updates today. Can I take

https://github.com/henryiii/uhi/blob/343fae1a7aa4e88e8a2d64016d897b58c6e2d260/uhi/typing/plottable.py

to be the standard to adhere to? (It's the version of plottable.py in this PR, after the only commit: 343fae1.

[henryiii]

Just waiting on a sign-off from @HDembinski so we are all in accord. We changed the meaning of "counts" to be effective counts" without changing the name, so I think the new name is likely fine.