[8.18] fix anchors (#125589)

docs/reference/modules/node.asciidoc

@@ -46,6 +46,29 @@ The following additional roles are available:
 
 * `voting_only`
 
+[NOTE]
+[[coordinating-only-node]]
+.Coordinating node
+===============================================
+
+Requests like search requests or bulk-indexing requests may involve data held
+on different data nodes. A search request, for example, is executed in two
+phases which are coordinated by the node which receives the client request --
+the _coordinating node_.
+
+In the _scatter_ phase, the coordinating node forwards the request to the data
+nodes which hold the data. Each data node executes the request locally and
+returns its results to the coordinating node. In the _gather_ phase, the
+coordinating node reduces each data node's results into a single global
+result set.
+
+Every node is implicitly a coordinating node. This means that a node that has
+an explicit empty list of roles in the `node.roles` setting will only act as a coordinating
+node, which cannot be disabled. As a result, such a node needs to have enough
+memory and CPU in order to deal with the gather phase.
+
+===============================================
+
 [IMPORTANT]
 ====
 If you set `node.roles`, ensure you specify every node role your cluster needs.