Rename ProneTime to Duration

More in line with our property naming conventions.

Additionally, added descs to ProneOffset and
ProneSequencePrefix, since at  least the purpose of
the former isn't entirely clear without looking at the code.

OpenRA.Mods.Common/UpdateRules/UpdatePath.cs

@@ -143,6 +143,7 @@ namespace OpenRA.Mods.Common.UpdateRules
 				new RemoveYesNo(),
 				new RemoveInitialFacingHardcoding(),
 				new RemoveAirdropActorTypeDefault(),
+				new RenameProneTime(),
 			})
 		};