[branding] Document slideshow options

src/branding/default/branding.desc

@@ -141,8 +141,23 @@ images:
 
 # The slideshow is displayed during execution steps (e.g. when the
 # installer is actually writing to disk and doing other slow things).
+#
+# The slideshow can be a QML file (recommended) which can display
+# arbitrary things -- text, images, animations, or even play a game --
+# during the execution step. The QML **is** abruptly stopped when the
+# execution step is done, though, so maybe a game isn't a great idea.
+#
+# The slideshow can also be a sequence of images (not recommended unless
+# you don't want QML at all in your Calamares). The images are displayed
+# at a rate of 1 every 2 seconds during the execution step.
+#
+# To configure a QML file, list a single filename:
+#   slideshow:               "show.qml"
+# To configure images, like the filenames (here, as an inline list):
+#   slideshow: [ "/etc/calamares/slideshow/0.png", "/etc/logo.png" ]
 slideshow:               "show.qml"
-# There are two available APIs for the slideshow:
+
+# There are two available APIs for a QML slideshow:
 #  - 1 (the default) loads the entire slideshow when the installation-
 #      slideshow page is shown and starts the QML then. The QML
 #      is never stopped (after installation is done, times etc.
@@ -151,6 +166,8 @@ slideshow:               "show.qml"
 #      onLeave() in the root object. After the installation is done,
 #      the show is stopped (first by calling onLeave(), then destroying
 #      the QML components).
+#
+# An image slideshow does not need to have the API defined.
 slideshowAPI: 2