Fix use-package :custom-face to set face-defface-spec (bug#77928)

Fix use-package :custom-face to set face-defface-spec (bug#77928)

By default, `face-set-spec' sets the override face spec, so face
attributes are combined with defaults rather than replacing them.
This was a behavior change that was an apparently unintended
consequence of commit 6b344a9.

Also set the `face-modified' property, which causes Customize to
flag the face as changed outside Customize.

* doc/misc/use-package.texi (Faces): Document the behavior.
* lisp/use-package/use-package-core.el (use-package-handler/:custom-face):
(use-package): Improve docstring to reflect implementation.
* test/lisp/use-package/use-package-tests.el
(use-package-test/:custom-face-1): (use-package-test/:custom-face-2):
(use-package-test/:custom-face-3): (use-package-test/:custom-face-4):
Add tests.

(cherry picked from commit ebcde0f90f67852d485a36941b0661cfd1b892eb)