DOC: Document NPY_RELAXED_STRIDES_CHECKING changes

This includes documentation in the release notes, as well as the
reference guide and smaller corrections. Thanks to Nathaniel
for major rewriting this.
DOC: Add NPY_RELAXED_STRIDES_DOCUMENTATION doc to ndarry.flags
Merge pull request #3221 from seberg/document-relaxed-strides

DOC: Document NPY_RELAXED_STRIDES_CHECKING changes