Document a few atmos classes.

Content.Server/Atmos/TileAtmosphere.cs

@@ -1,3 +1,4 @@
+using Content.Server.Atmos.EntitySystems;
 using Content.Shared.Atmos;
 using Content.Shared.Maps;
 using Robust.Shared.Map;
@@ -8,6 +9,7 @@ namespace Content.Server.Atmos
 {
     /// <summary>
     ///     Internal Atmos class that stores data about the atmosphere in a grid.
+    ///     You shouldn't use this directly, use <see cref="AtmosphereSystem"/> instead.
     /// </summary>
     public class TileAtmosphere : IGasMixtureHolder
     {