Add clarification on expr_form usage and future deprecation (#37964)

doc/ref/peer.rst

@@ -124,3 +124,7 @@ To match minions using other matchers, use ``expr_form``:
 .. code-block:: bash
 
     # salt-call publish.publish 'webserv* and not G@os:Ubuntu' test.ping expr_form='compound'
+
+.. note::
+    The expr_form argument will be renamed to ``tgt_type`` in the Nitrogen
+    release of Salt.

doc/topics/mine/index.rst

@@ -179,3 +179,7 @@ to add them to the pool of load balanced servers.
     {% endfor %}
 
     <...file contents snipped...>
+
+.. note::
+    The expr_form argument will be renamed to ``tgt_type`` in the Nitrogen
+    release of Salt.

doc/topics/reactor/index.rst

@@ -376,6 +376,13 @@ Use the ``expr_form`` argument to specify a matcher:
         - arg:
           - rm -rf /tmp/*
 
+.. note::
+    An easy mistake to make here is to use ``tgt_type`` instead of
+    ``expr_form``, since the job cache and events all refer to the targeting
+    method as ``tgt_type``. As of the Nitrogen release of Salt, ``expr_form``
+    will be deprecated in favor of using ``tgt_type``, to help with this
+    confusion.
+
 Any other parameters in the :py:meth:`LocalClient().cmd()
 <salt.client.LocalClient.cmd>` method can be specified as well.