MAINT: stats: Add note about change to scoreatpercentile in release 0.12.0.

Also make a test of scoreatpercentile on a 2D array more explicit
about the expected result.

Closes gh-3329.
Merge pull request #4058 from WarrenWeckesser/maint-scoreatpercentile

DOC: stats: Add note about change to scoreatpercentile in release 0.12.0.