[displaymanager] Document *defaultDesktopEnvironment*

- while the keys are mandatory, their meaning is entirely optional.

src/modules/displaymanager/displaymanager.conf

@@ -13,7 +13,21 @@ displaymanagers:
   - lxdm
   - kdm
 
-#Enable the following settings to force a desktop environment in your displaymanager configuration file:
+# Enable the following settings to force a desktop environment
+# in your displaymanager configuration file. This will attempt
+# to configure the given DE (without checking if it is installed).
+# The DM configuration for each potential DM may **or may not**
+# support configuring a default DE, so the keys are mandatory
+# but their interpretation is up to the DM configuration.
+#
+# Subkeys of *defaultDesktopEnvironment* are:
+#  - *executable* a full path to an executable (manda)
+#  - *desktopFile* a .desktop filename (optional)
+#
+# If this is **not** set, then Calamares will look for installed
+# DE's and pick the first one it finds.
+#
+
 #defaultDesktopEnvironment:
 #    executable: "startkde"
 #    desktopFile: "plasma"