[3.15] gh-153528: Document mode="func_type" in compile() (GH-153561) (#160479)

(cherry picked from commit 02c5d55)

Co-authored-by: Pranav Choudhary <pranav.nstblr@gmail.com>

Author: miss-islington

Doc/builtins/functions.rst

@@ -314,6 +314,10 @@ are always available.  They are listed here in alphabetical order.
    consists of a single expression, or ``'single'`` if it consists of a single
    interactive statement (in the latter case, expression statements that
    evaluate to something other than ``None`` will be printed).
+   It can also be ``'func_type'`` if *source* consists of a
+   function type signature, for example ``(int) -> str``; see
+   :func:`ast.parse` for more information. This mode requires passing
+   :data:`ast.PyCF_ONLY_AST` in *flags*.
 
    The optional arguments *flags* and *dont_inherit* control which
    :ref:`compiler options <ast-compiler-flags>` should be activated
@@ -387,6 +391,9 @@ are always available.  They are listed here in alphabetical order.
       ``ast.PyCF_ALLOW_TOP_LEVEL_AWAIT`` can now be passed in flags to enable
       support for top-level ``await``, ``async for``, and ``async with``.
 
+   .. versionchanged:: 3.8
+      Added support for ``mode='func_type'``.
+
    .. versionchanged:: 3.12
       :exc:`SyntaxError` is raised instead of :exc:`ValueError` when null bytes
       are encountered in *source*.