Review StyleCop rules.

- Enforce SA1604 ElementDocumentationShouldHaveSummary.
- Enforce SA1629 DocumentationTextShouldEndWithAPeriod.
- Turn off some rules covered by IDExxxx rules.
- Remaining rules are treated as part of OpenRA style.

OpenRA.Game/Player.cs

@@ -253,7 +253,7 @@ namespace OpenRA
 			return PlayerRelationship.Neutral;
 		}
 
-		/// <summary> returns true if player is null </summary>
+		/// <summary>Returns true if player is null.</summary>
 		public bool IsAlliedWith(Player p)
 		{
 			return RelationshipWith(p) == PlayerRelationship.Ally;