SOLR-12387: added documentation

solr/solr-ref-guide/src/collections-api.adoc

@@ -1085,6 +1085,48 @@ http://localhost:8983/solr/admin/collections?action=CLUSTERPROP&name=urlScheme&v
 </response>
 ----
 
+=== Deeply Nested Cluster Properties ===
+
+==== `collectionDefaults` ====
+It is possible to set cluster-wide default values for certain attributes of a collection.
+
+
+*Example 1: Set/update default values*
+[source]
+----
+curl -X POST -H 'Content-type:application/json' --data-binary '
+{ "set-obj-property" : {
+    "collectionDefaults" : {
+        "numShards" : 2,
+        "nrtReplicas" : 1,
+        "tlogReplicas" : 1,
+        "pullReplicas" : 1,
+
+   }
+}' http://localhost:8983/api/cluster
+----
+
+*Example 2: Unset the value of `nrtReplicas` alone*
+[source]
+----
+curl -X POST -H 'Content-type:application/json' --data-binary '
+{ "set-obj-property" : {
+    "collectionDefaults" : {
+        "nrtReplicas" : null,
+   }
+}' http://localhost:8983/api/cluster
+----
+
+*Example 2: Unset all values in `collectionDefaults`*
+[source]
+----
+curl -X POST -H 'Content-type:application/json' --data-binary '
+{ "set-obj-property" : {
+    "collectionDefaults" : null
+}' http://localhost:8983/api/cluster
+----
+
+
 [[collectionprop]]
 == COLLECTIONPROP: Collection Properties