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/FileSystem/FileSystem.cs

@@ -271,7 +271,7 @@ namespace OpenRA.FileSystem
 		}
 
 		/// <summary>
-		/// Returns true if the given filename references an external mod via an explicit mount
+		/// Returns true if the given filename references an external mod via an explicit mount.
 		/// </summary>
 		public bool IsExternalModFile(string filename)
 		{