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/Input/HotkeyReference.cs

@@ -14,7 +14,7 @@ using System;
 namespace OpenRA
 {
 	/// <summary>
-	/// A reference to either a named hotkey (defined in the game settings) or a statically assigned hotkey
+	/// A reference to either a named hotkey (defined in the game settings) or a statically assigned hotkey.
 	/// </summary>
 	public class HotkeyReference
 	{