gh-95555: Fix a stale comment about negating \P in a character class

GH-152245 lifted the restriction that a negated multi-range property
could not appear inside a character class, and added assertions in
test_property_escapes showing that [\P{ASCII}] and friends now match.
It left in place an earlier comment in the same test saying that,
unlike an engine category, \P of a multi-range property cannot be
negated inside a character class.

The distinction that remains is how the member is compiled, not
whether it is allowed: an engine category joins the set directly as a
CATEGORY, while a multi-range \P is alternated in as a separate branch.

Author: fedonman

Lib/test/test_re.py

@@ -1010,8 +1010,8 @@ def test_property_escapes(self):
             with self.subTest(char=c):
                 self.assertEqual(bool(ci.fullmatch(c)), expect)
         self.assertTrue(re.fullmatch(r'\p{Alphabetic=No}+', '123 '))
-        # These are engine categories, so (unlike \P of a multi-range
-        # property) they can be negated inside a character class.
+        # These are engine categories, so a negated one joins the set directly
+        # as a CATEGORY rather than being alternated in as a separate branch.
         self.assertTrue(re.fullmatch(r'[\P{Alphabetic}]+', '123 .'))
         self.assertTrue(re.fullmatch(r'[\p{XID_Start}_]+', 'foo_bar'))