Fix wording on node roles (elastic#123939)

In the 8.17 docs there is a statement that intends to say that
nodes without roles are coordinating-only nodes. However, the wording
was misleading because it referred to  leaving the setting _unset_
rather than explictly setting it to _empty_.

Author: tvernum

docs/reference/modules/node.asciidoc

@@ -46,7 +46,8 @@ The following additional roles are available:
 
 * `voting_only`
 
-[[coordinating-only-node]]If you leave `node.roles` unset, then the node is considered to be a <<coordinating-only-node-role,coordinating only node>>.
+[[coordinating-only-node]]If you set `node.roles` to an empty array (`node.roles: [ ]`),
+then the node is considered to be a <<coordinating-only-node-role,coordinating only node>>.
 
 [IMPORTANT]
 ====
@@ -155,4 +156,4 @@ including:
 
 * <<cluster-name,`cluster.name`>>
 * <<node-name,`node.name`>>
-* <<modules-network,network settings>>
+* <<modules-network,network settings>>