Add an example for map() to the filters page

docsite/rst/playbooks_filters.rst

@@ -527,11 +527,18 @@ To escape special characters within a regex, use the "regex_escape" filter::
 
     # convert '^f.*o(.*)$' to '\^f\.\*o\(\.\*\)\$'
     {{ '^f.*o(.*)$' | regex_escape() }}
+    
+To make use of one attribute from each item in a list of complex variables, use the "map" filter (see the `Jinja2 map() docs`_ for more)::
+
+    # get a comma-separated list of the mount points (e.g. "/,/mnt/stuff") on a host
+    {{ ansible_mounts|map(attribute='mount')|join(',') }}
 
 A few useful filters are typically added with each new Ansible release.  The development documentation shows
 how to extend Ansible filters by writing your own as plugins, though in general, we encourage new ones
 to be added to core so everyone can make use of them.
 
+.. _Jinja2 map() docs: http://jinja.pocoo.org/docs/dev/templates/#map
+
 .. _builtin filters: http://jinja.pocoo.org/docs/templates/#builtin-filters
 
 .. seealso::