Add comment on DEALLOCATED being state eventual (#1622) (#1623)

There is a known situation in which a previously `DEALLOCATED` offline
server will transiently show as `DEALLOCATING` when restarting. In
short, it doesn't know it has had it's user databases allocated away so
starts them up, but it will eventually go back to `DEALLOCATED`. This PR
makes this clear and updates the server state diagram.

---------
Cherry-picked from #1622

Co-authored-by: Jack Waudby <[email protected]>

Author: renetapopova

modules/ROOT/images/server-states4.png

@@ -19,7 +19,7 @@ A server can exist in five different states within the DBMS:
 * Dropped
 
 
-image::server-states3.png[width=800]
+image::server-states4.png[width=800]
 
 === Free state
 
@@ -88,8 +88,9 @@ Additionally, servers which are deallocating will not have any further databases
 [role=label--new-5.15]
 === Deallocated state
 
-When a server is in the deallocated state it no longer hosts any databases any databases besides system and can be removed from the cluster.
+When a server is in the deallocated state, it no longer hosts any databases besides `system` and can be removed from the cluster.
 Additionally, deallocated servers cannot have any further databases allocated to them.
+Note that there is a known situation in which a previously deallocated offline server can transiently show as deallocating when restarting, it will, however, eventually return to the deallocated state without intervention.
 
 === Cordoned state