Backport 7363, ENH: Make no unshare mask future warnings less noisy

Incorporates Nathaniels suggestions for a longer explanation
in the release notes.

Removed superfluous `ceil` call in automated bin width estimator.
Updated tests to reflect modified estimator.

Described ad nauseum the relationship between `range` parameter and bin estimation.
Updated formulas for estimators now that they are returning bin widths.

Fixes #7411. Tests and documentation updated.
Fixes other small issues with range and bin count computations.

This doesn't actually test much, as we don't have any inputs where that
was not already the case. But at least it is there and perhaps a fuzz
test can be added at a later date.

The non-nan elements of the result of corrcoef should satisfy the
inequality abs(x) <= 1 and the non-nan elements of the diagonal should
be exactly one. We can't guarantee those results due to roundoff, but
clipping the real and imaginary parts to the interval [-1, 1] improves
things to a small degree.

Closes #7392.

Backport 7407, BUG: Fix decref before incref for in-place accumulate

closes gh-7402