T S1: added an explanation of turns

data/campaigns/tutorial/scenarios/01_Tutorial_part_1.cfg

@@ -428,7 +428,14 @@
 
         [allow_end_turn][/allow_end_turn]
 
-        {PRINT (_"Click on the <b>End Turn</b> button in the bottom right of the screen to end your turn")}
+        [message]
+            speaker=narrator
+            caption= _"Turns"
+            image=wesnoth-icon.png
+            message= _"Every turn, each side in a scenario gets a chance to make their move. Once you've completed everything you wish to do this turn, click on the <b>End Turn</b> button in the bottom right of the screen. The other teams, whether controlled by the AI or other human players, will then make their move. Some scenarios must be completed in a certain number of turns. You can see what turn it is, and any applicable turn limit, next to the flag icon at the top of the screen."
+        [/message]
+
+        {PRINT (_"End your turn")}
     [/event]
 
     # TURN 2: lesson in healing