Settings: document new disable-cancel-during-exec

CHANGES

@@ -22,7 +22,9 @@ This release contains contributions from (alphabetically by first name):
 
  * Under-the-hood code cleanups in lots of parts of the core. Calamares now
    builds without warnings when Clang 8 is used.
-
+ * A new *disable-cancel-during-exec* setting provides more fine-grained
+   control than *disable-cancel* (which hides the button entirely).
+   
 ## Modules ##
 
  * All of the Python-based modules now have translations enabled.

settings.conf

@@ -155,6 +155,8 @@ dont-chroot: false
 # oem-setup: true
 
 # If this is set to true, the "Cancel" button will be disabled entirely.
+# The button is also hidden from view.
+#
 # This can be useful if when e.g. Calamares is used as a post-install 
 # configuration tool and you require the user to go through all the 
 # configuration steps.
@@ -167,4 +169,8 @@ disable-cancel: false
 # If this is set to true, the "Cancel" button will be disabled once
 # you start the 'Installation', meaning there won't be a way to cancel
 # the Installation until it has finished or installation has failed.
+#
+# Default is false, but Calamares will complain if this is not explicitly set.
+#
+# YAML: boolean.
 disable-cancel-during-exec: false