Add a str class entry to the "Text Sequence Type" section (issue #16209).

This commit also moves the documentation for the str built-in function to
the new class entry.  Links to :class:`str` now go to the class entry with
the string methods immediately afterwards.
diff --git a/Misc/NEWS b/Misc/NEWS
index 62193b9..cd77da8 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -292,6 +292,9 @@
 Documentation
 -------------
 
+- Issue #16209: Move the documentation for the str built-in function to a new
+  str class entry in the "Text Sequence Type" section.
+
 - Issue #13538: Improve str() and object.__str__() documentation.
 
 - Issue #16489: Make it clearer that importlib.find_loader() requires any and