Draft API changes to api/v1/fleet/hosts/{id} endpoint for software inventory (#669)

Update documentation for the `api/v1/fleet/hosts/{id}` endpoint to align with the first iteration of Software inventory #399 in Fleet.

docs/1-Using-Fleet/3-REST-API.md

@@ -587,7 +587,9 @@ None.
 
 ### Get host
 
-Returns the information of the specified host.
+Returns the information of the specified host. 
+
+The endpoint returns the host's installed `software` if the software inventory feature flag is turned on. This feature flag is turned off by default. [Check out the feature flag documentation](../2-Deployment/2-Configuration.md#feature-flags) for instructions on how to turn on the software inventory feature.
 
 `GET /api/v1/fleet/hosts/{id}`
 
@@ -644,7 +646,51 @@ Returns the information of the specified host.
         "additional": {},
         "enroll_secret_name": "bar",
         "status": "offline",
-        "display_text": "259404d30eb6"
+        "display_text": "259404d30eb6",
+        "labels": [
+          {
+            "created_at": "2021-01-14T16:37:24Z",
+            "updated_at": "2021-01-14T16:37:24Z",
+            "id": 6,
+            "name": "All Hosts",
+            "description": "All hosts which have enrolled in Fleet",
+            "query": "select 1;",
+            "label_type": "builtin",
+            "label_membership_type": "dynamic"
+          },
+          {
+            "created_at": "2021-01-14T16:37:24Z",
+            "updated_at": "2021-01-14T16:37:24Z",
+            "id": 7,
+            "name": "macOS",
+            "description": "All macOS hosts",
+            "query": "select 1 from os_version where platform = 'darwin';",
+            "label_type": "builtin",
+            "label_membership_type": "dynamic"
+          }
+        ],
+        "packs": [
+          {
+            "created_at": "2021-01-17T00:02:35Z",
+            "updated_at": "2021-01-17T00:02:35Z",
+            "id": 1,
+            "name": "osquery_monitoring"
+          }
+        ],
+        "software": [
+          {
+            "id": 1,
+            "name": "CentOS Linux $releasever - AppStream",
+            "version": "",
+            "source": "yum_packages",
+          },
+          {
+            "id": 2,
+            "name": "curl",
+            "version": "7.61.1",
+            "source": "rpm_packages",
+          },
+        ]
     }
 }
 ```