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.

1 files changed, 6 insertions, 1 deletions

diff --git a/doc/misc/use-package.texi b/doc/misc/use-package.texi
index c8630cee1c9..2f980df9f45 100644
--- a/doc/misc/use-package.texi
+++ b/doc/misc/use-package.texi
@@ -1471,7 +1471,7 @@ faces.  Example:
 (use-package example
   :custom-face
   (example-1-face ((t (:foreground "LightPink"))))
-  (example-2-face ((t (:foreground "LightGreen"))) face-defspec-spec))
+  (example-2-face ((t (:foreground "LightGreen")))))
 @end group
 
 @group
@@ -1486,6 +1486,11 @@ faces.  Example:
 @end group
 @end lisp
 
+Similarly to @code{:custom} (@pxref{User options}), this allows
+configuring customizable faces outside of Customize (@pxref{Saving
+Customizations,,, emacs, GNU Emacs Manual}).  Using both systems to
+configure the same face can lead to confusing results.
+
 @node Hiding minor modes
 @section Hiding minor modes with diminish and delight
 @cindex hiding minor modes