BUG: display.precision options seems off-by-one  (GH10451)
Closes #10451 

I made a call here in response to my questions on #10451 and hopefully people like it. I made it clear that "precision" refers to places after the decimal, not significant figures, and changed the default value to match so that for many pandas users no change would be detected. I updated the Options docs and also What's New. For tests I basically updated the precision setting to the new semantics, so that the expected strings wouldn't need to change.

The one question I have is the code that computes `too_long`. This compares the longest formatted string against what looks to be an arbitrary constant of "number of digits + 5". Changing the 5 to a 4 or 6 doesn't trip up any unit tests. If it's desired, this could be increased by 1 as the "new" digits value is effectively 1 less than what the old value in terms of its effects on formatting, so the 5 would need to change to a 6 to maintain behavior. I could write tests on this as well.
