Make headerregistry fully part of the provisional api.

When I made the checkin of the provisional email policy, I knew that
Address and Group needed to be made accessible from somewhere.  The more
I looked at it, though, the more it became clear that since this is a
provisional API anyway, there's no good reason to hide headerregistry as
a private API.  It was designed to ultimately be part of the public API,
and so it should be part of the provisional API.

This patch fully documents the headerregistry API, and deletes the
abbreviated version of those docs I had added to the provisional policy
docs.
diff --git a/Lib/test/test_email/test_policy.py b/Lib/test/test_email/test_policy.py
index 45a7666..983bd49 100644
--- a/Lib/test/test_email/test_policy.py
+++ b/Lib/test/test_email/test_policy.py
@@ -5,7 +5,7 @@
 import email.policy
 import email.parser
 import email.generator
-from email import _headerregistry
+from email import headerregistry
 
 def make_defaults(base_defaults, differences):
     defaults = base_defaults.copy()
@@ -185,11 +185,11 @@
     def test_default_header_factory(self):
         h = email.policy.default.header_factory('Test', 'test')
         self.assertEqual(h.name, 'Test')
-        self.assertIsInstance(h, _headerregistry.UnstructuredHeader)
-        self.assertIsInstance(h, _headerregistry.BaseHeader)
+        self.assertIsInstance(h, headerregistry.UnstructuredHeader)
+        self.assertIsInstance(h, headerregistry.BaseHeader)
 
     class Foo:
-        parse = _headerregistry.UnstructuredHeader.parse
+        parse = headerregistry.UnstructuredHeader.parse
 
     def test_each_Policy_gets_unique_factory(self):
         policy1 = email.policy.EmailPolicy()
@@ -197,10 +197,10 @@
         policy1.header_factory.map_to_type('foo', self.Foo)
         h = policy1.header_factory('foo', 'test')
         self.assertIsInstance(h, self.Foo)
-        self.assertNotIsInstance(h, _headerregistry.UnstructuredHeader)
+        self.assertNotIsInstance(h, headerregistry.UnstructuredHeader)
         h = policy2.header_factory('foo', 'test')
         self.assertNotIsInstance(h, self.Foo)
-        self.assertIsInstance(h, _headerregistry.UnstructuredHeader)
+        self.assertIsInstance(h, headerregistry.UnstructuredHeader)
 
     def test_clone_copies_factory(self):
         policy1 = email.policy.EmailPolicy()