Introduce the term "node" (#2793)

Node appears throughout the documentation. Helpful to expressly place the term "node" with the hierarchal tree/series to better illuminate concepts like groupByNode.

docs/terminology.rst

@@ -18,6 +18,9 @@ terms mean in the context of Graphite.
 
     metric series
       See :term:`series`
+
+    node
+      An element of the name of a :term:`series` separated by periods (``.``).
 
     precision
       See :term:`resolution`
@@ -35,7 +38,7 @@ terms mean in the context of Graphite.
 
     series
       A named set of datapoints. A series is identified by a unique name, which is composed of
-      elements separated by periods (``.``) which are used to display the collection of series
+      elements (each, a :term:`node`) separated by periods (``.``) which are used to display the collection of series
       into a hierarchical tree. A series storing system load average on a server called ``apache02``
       in datacenter ``metro_east`` might be named as ``metro_east.servers.apache02.system.load_average``