bpo-36502: Correct documentation of str.isspace() (GH-15019)

bpo-36502: Correct documentation of str.isspace() (GH-15019)

The documented definition was much broader than the real one:
there are tons of characters with general category "Other",
and we don't (and shouldn't) treat most of them as whitespace.

Rewrite the definition to agree with the comment on
_PyUnicode_IsWhitespace, and with the logic in makeunicodedata.py,
which is what generates that function and so ultimately governs.

Add suitable breadcrumbs so that a reader who wants to pin down
exactly what this definition means (what's a "bidirectional class"
of "B"?) can do so.  The `unicodedata` module documentation is an
appropriate central place for our references to Unicode's own copious
documentation, so point there.

Also add to the isspace() test a thorough check that the
implementation agrees with the intended definition.