whatsnew-3.10: Mention PEP 647 in the Release highlights section. (pythonGH-99853)

Mention PEP 647 in the Release highlights section.

Also re-ordered the list so it matches the order in the details sections below.
(cherry picked from commit d74a588)

Co-authored-by: Yilei "Dolee" Yang <[email protected]>

Author: miss-islington

Doc/whatsnew/3.10.rst

@@ -77,8 +77,9 @@ Interpreter improvements:
 New typing features:
 
 * :pep:`604`, Allow writing union types as X | Y
-* :pep:`613`, Explicit Type Aliases
 * :pep:`612`, Parameter Specification Variables
+* :pep:`613`, Explicit Type Aliases
+* :pep:`647`, User-Defined Type Guards
 
 Important deprecations, removals or restrictions: