Optionally propagate pillar data from Salt to reclass

Optionally, data from pillars that run before the reclass ``ext_pillar``
(i.e. Salt's builtin ``pillar_roots``, as well as other ``ext_pillar``
modules listed before the ``reclass_adapter``) can be made available to
reclass. Please use this with caution as referencing data from Salt in
the inventory will make it harder or impossible to run |reclass| in
other environments. This feature is therefore turned off by default and
must be explicitly enabled in the Salt master configuration file, like
this:

  reclass: &reclass
      […]
      propagate_pillar_data_to_reclass: True

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/doc/source/changelog.rst b/doc/source/changelog.rst
index 25bda17..9f91569 100644
--- a/doc/source/changelog.rst
+++ b/doc/source/changelog.rst
@@ -5,6 +5,8 @@
 ========= ========== ========================================================
 Version   Date       Changes
 ========= ========== ========================================================
+                     * Salt: pillar data from previous pillars are now
+                       available to reclass parameter interpolation
                      * yaml_fs: classes may be defined in subdirectories
                        (closes: #12, #19, #20)
                      * Migrate Salt adapter to new core API (closes: #18)
diff --git a/doc/source/salt.rst b/doc/source/salt.rst
index f0cc733..a5f9a73 100644
--- a/doc/source/salt.rst
+++ b/doc/source/salt.rst
@@ -178,9 +178,9 @@
 Similarly, all parameters that are collected and merged eventually end up in
 the pillar data of a specific node.
 
-However, the pillar data of a node include all the information about classes
-and applications, so you could theoretically use them to target your Salt
-calls at groups of nodes defined in the |reclass| inventory, e.g.
+The pillar data of a node include all the information about classes and
+applications, so you could theoretically use them to target your Salt calls at
+groups of nodes defined in the |reclass| inventory, e.g.
 
 ::
 
@@ -189,5 +189,17 @@
 Unfortunately, this does not work yet, please stay tuned, and let me know
 if you figure out a way. `Salt issue #5787`_ is also of relevance.
 
+Optionally, data from pillars that run before the |reclass| ``ext_pillar``
+(i.e. Salt's builtin ``pillar_roots``, as well as other ``ext_pillar`` modules
+listed before the ``reclass_adapter``) can be made available to |reclass|.
+Please use this with caution as referencing data from Salt in the inventory
+will make it harder or impossible to run |reclass| in other environments. This
+feature is therefore turned off by default and must be explicitly enabled in
+the Salt master configuration file, like this::
+
+  reclass: &reclass
+      […]
+      propagate_pillar_data_to_reclass: True
+
 .. include:: substs.inc
 .. include:: extrefs.inc