Document Single Item Array filter

Author: kpaulisse

doc/advanced-filter.md

@@ -11,6 +11,7 @@ Here is the list of available filters and an explanation of each:
 
 - [Absent File](/doc/advanced-filter.md#absent-file) - Ignore parameter changes of a file that is declared to be absent
 - [JSON](/doc/advanced-filter.md#json) - Ignore whitespace differences if JSON parses to the same object
+- [SingleItemArray](/doc/advanced-filter.md#SingleItemArray) - Ignore differences between object and array containing only that object
 - [YAML](/doc/advanced-filter.md#yaml) - Ignore whitespace/comment differences if YAML parses to the same object
 
 ## Absent File
@@ -84,6 +85,28 @@ If a file resource has extension `.json` and a difference in its content is obse
 
 This allows you to ignore changes in whitespace, comments, etc., that are not meaningful to a machine parsing the file. Note that changes to files may still trigger Puppet to restart services even though these changes are not displayed in the octocatalog-diff output.
 
+## Single Item Array
+
+#### Usage
+
+```
+--filters SingleItemArray
+```
+
+#### Description
+
+When enabling the future parser or upgrading between certain versions of Puppet, the internal structure of the catalog for certain parameters can change as shown in the following example:
+
+```
+Old: { "notify": "Service[foo]" }
+New: { "notify": [ "Service[foo]" ] }
+```
+
+This filter will suppress differences for the value of a parameter when:
+
+- The value in one catalog is an object, AND
+- The value in the other catalog is an array containing *only* that same object
+
 ## YAML
 
 #### Usage