doc: Mention get_node in Node.get_child documentation

See #22052.

doc/classes/Node.xml

@@ -185,6 +185,7 @@
 			</argument>
 			<description>
 				Returns a child node by its index (see [method get_child_count]). This method is often used for iterating all children of a node.
+				To access a child node via its name, use [method get_node].
 			</description>
 		</method>
 		<method name="get_child_count" qualifiers="const">