Documenting second parameter of the default filter (#31184)

* Documenting second parameter of the default filter

* Edited for clarity and grammar.

* Adding missing colon

docs/docsite/rst/playbooks_filters.rst

@@ -63,13 +63,18 @@ The variable value will be used as is, but the template evaluation will raise an
 Defaulting Undefined Variables
 ``````````````````````````````
 
-Jinja2 provides a useful 'default' filter, that is often a better approach to failing if a variable is not defined::
+Jinja2 provides a useful 'default' filter that is often a better approach to failing if a variable is not defined::
 
     {{ some_variable | default(5) }}
 
 In the above example, if the variable 'some_variable' is not defined, the value used will be 5, rather than an error
 being raised.
 
+If the variable evaluates to an empty string, the second parameter of the filter should be set to
+`true`::
+
+    {{ lookup('env', 'MY_USER') | default('admin', true) }}
+
 
 .. _omitting_undefined_variables: