Update description of Stats - Hourly Breakdown

[Danika-Dakika]

[The Stats page needs quite a few updates, but I'd encourage pull requests to address them one-at-a-time.]

Here's a good one:
https://docs.ankiweb.net/stats.html#the-graphs
This pretty clearly describes the old Stats graph, not the current one.

Hourly Breakdown
This graph shows what percentage of total reviews you have passed (i.e., not pressed Again on) during given hours. The larger, darker bars and left axis show the success rate; the thinner, lighter bars and right axis show the number of reviews you’ve made at that hour (so you know how significant the results are).

Old:

New: