doc.glossary: add idempotent

doc/glossary.rst

@@ -73,6 +73,17 @@ Glossary
         The collection of states to be applied to a system. *See also*:
         :ref:`state layers <state-layers-highstate>`.
 
+    Idempotent
+        An action that ensures the system is in a well-known state regardless
+        of the system's state before the action is applied.  A corollary to
+        this is that applying the action multiple times results in no changes
+        to the system.  State module functions should be idempotent.  Some
+        state module functions, such as :mod:`cmd.run <salt.states.cmd.run>`
+        are not idempotent by default but can be made idempotent with the
+        proper use of requisites such as :ref:```unless`` <unless-requisite>`
+        and :ref:```onlyif`` <onlyif-requisite>`.  For more information, *see*
+        `wikipedia <https://en.wikipedia.org/wiki/Idempotent>`_.
+
     Jinja
         A templating language which allows variables and simple logic to be
         dynamically inserted into static text files when they are rendered.

doc/ref/states/requisites.rst

@@ -592,6 +592,8 @@ Reload
 after a state finishes. ``reload_pillar`` and ``reload_grains`` can also be set.
 See :ref:`Reloading Modules <reloading-modules>`.
 
+.. _unless-requisite:
+
 Unless
 ------
 
@@ -640,6 +642,8 @@ For example:
 In the above case, ``some_check`` will be run prior to _each_ name -- once for
 ``first_deploy_cmd`` and a second time for ``second_deploy_cmd``.
 
+.. _onlyif-requisite:
+
 Onlyif
 ------