Document Host object's name field

While being present in the attribute list for the Service object, the
Host's name field is missing in the list of all attributes, even when
being referred to in multiple examples of `assign where`.

It is also present in the REST API's response:
$ curl -s -k -u root:icinga 'https://localhost:5665/v1/objects/hosts' | jq '.results[0].name'

doc/09-object-types.md

@@ -364,6 +364,7 @@ Configuration Attributes:
   flapping\_ignore\_states  | Array                 | **Optional.** A list of states that should be ignored during flapping calculation. By default no state is ignored.
   volatile                  | Boolean               | **Optional.** Treat all state changes as HARD changes. See [here](08-advanced-topics.md#volatile-services-hosts) for details. Defaults to `false`.
   zone                      | Object name           | **Optional.** The zone this object is a member of. Please read the [distributed monitoring](06-distributed-monitoring.md#distributed-monitoring) chapter for details.
+  name                      | String                | **Required.** The host name. Must be unique on a per-host basis. For advanced usage in [apply rules](03-monitoring-basics.md#using-apply) only.
   command\_endpoint         | Object name           | **Optional.** The endpoint where commands are executed on.
   notes                     | String                | **Optional.** Notes for the host.
   notes\_url                | String                | **Optional.** URL for notes for the host (for example, in notification commands).