Improve config file documentation

doc/katarakt.txt

@@ -47,6 +47,9 @@ CONFIGURATION
 Variables and key bindings can be changed by modifying the katarakt.ini file.
 By default it is read from '$XDG_CONFIG_HOME'.
 
+Variables must be listed under the section "[Settings]", keys under section
+"[Keys]".  For a list of all variables with a short explanation see below.
+
 KEY BINDINGS
 ------------
 *q* ::

src/config.cpp

@@ -40,7 +40,7 @@ CFG::CFG() :
 	tmp_values["start_page"] = 0;
 	tmp_values["fullscreen"] = false;
 
-	settings.beginGroup("keys");
+	settings.beginGroup("Keys");
 	// canvas keys
 	keys["set_presentation_layout"] = QStringList() << "1";
 	keys["set_grid_layout"] = QStringList() << "2";