MGMT-11404: add rest-api doc for cluster tags (#4252)

Added documentation for creating/updating cluster tags
using the rest-api (in docs/user-guide).

docs/user-guide/rest-api-cluster-tags.md

@@ -0,0 +1,54 @@
+# REST-API - Cluster Tags
+
+The `tags` property in cluster object is a comma-separated list of tags that are associated to the cluster. Each tag is a free-text string that can consist of the following characters:
+Alphanumeric (aA-zZ, 0-9), underscore (_) and white-spaces.
+
+Tags are not read or manipulated by the system, so they can be used for marking clusters with some filtering criteria, installation tracking indication, etc.
+
+## Usage
+
+* The property can be specified when creating (v2RegisterCluster) or updating (V2UpdateCluster) a cluster.
+* The tags are stored in the cluster object, so the property can be fetched when getting (v2GetCluster) the cluster.
+* The property can be cleared by specifying an empty string.
+
+
+## Examples
+
+### Create tags (using v2RegisterCluster)
+
+```bash
+cat register_cluster.json
+{
+    "name":"test",
+    "pull_secret":"<pull_secret>",
+    "openshift_version":"4.10",
+    "tags":"tag1,tag_2,tag 3"
+}
+```
+
+```bash
+curl -X POST -H "Content-Type: application/json" -d @register_cluster.json \
+    <HOST>:<PORT>/api/assisted-install/v2/clusters
+```
+
+### Update tags (using V2UpdateCluster)
+
+```bash
+cat update_cluster.json
+{
+    "tags":"tag1,tag_2,tag 3,tag4"
+}
+```
+
+```bash
+curl -X PATCH -H "Content-Type: application/json" -d @update_cluster.json \
+    <HOST>:<PORT>/api/assisted-install/v2/clusters/<cluster_id>
+```
+
+### Get tags (using v2GetCluster)
+
+```bash
+curl <HOST>:<PORT>/api/assisted-install/v2/clusters/<cluster_id> | jq '.tags'
+
+output: "tag1,tag_2,tag 3,tag4"
+```