Explain how namespaces work with allocations (#2257)

* Add section explaining namespaces in the allocation reference

* Clean up code

* Make documentation changes based on PR comments

Co-authored-by: Emily Nguyen <eminguyen@google.com>

site/content/en/docs/Reference/gameserverallocation.md

@@ -150,5 +150,10 @@ The `spec` field is the actual `GameServerAllocation` specification, and it is c
 - `metadata` is an optional list of custom labels and/or annotations that will be used to patch
   the game server's metadata in the moment of allocation. This can be used to tell the server necessary session data
 
-[gameserverselector]: {{% ref "/docs/Reference/agones_crd_api_reference.html#allocation.agones.dev/v1.GameServerSelector"  %}}
+Each `GameServerAllocation` will allocate from a single [namespace][namespace]. The namespace can be specified outside of
+the spec, either with the `--namespace` flag when using the command line / `kubectl` or
+[in the url]({{% ref "/docs/Guides/access-api.md#allocate-a-gameserver-from-a-fleet-named-simple-game-server-with-gameserverallocation"  %}})
+when using an API call. If not specified when using the command line, the [namespace][namespace] will be automatically set to `default`.
 
+[gameserverselector]: {{% ref "/docs/Reference/agones_crd_api_reference.html#allocation.agones.dev/v1.GameServerSelector"  %}}
+[namespace]: https://kubernetes.io/docs/concepts/overview/working-with-objects/namespaces