minor #2870 document support for defining default argument values in the macro tag (olets)

This PR was merged into the 1.x branch.

Discussion
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document support for defining default argument values in the macro tag

The `macro` tag documentation reads

> Macros differ from native PHP functions in a few ways:
> - Default argument values are defined by using the default filter in the macro body

and provides the example

```twig
{% macro input(name, value, type, size) %}
    <input type="{{ type|default('text') }}" name="{{ name }}" value="{{ value|e }}" size="{{ size|default(20) }}" />
{% endmacro %}
```

The implication is that native PHP function-like default argument value definition is not supported.

v1.12 added support for

```twig
{% macro input1(name, value, type = 'text', size = 20) %}
    <input type="{{ type }}" name="{{ name }}" value="{{ value|e }}" size="{{ size }}" />
{% endmacro %}
```

This updates the documentation to cover both ways of defining default argument values.

Commits
-------

40c7553a document support for defining default argument values in the macro tag

doc/tags/macro.rst

@@ -13,10 +13,16 @@ Here is a small example of a macro that renders a form element:
         <input type="{{ type|default('text') }}" name="{{ name }}" value="{{ value|e }}" size="{{ size|default(20) }}" />
     {% endmacro %}
 
-Macros differ from native PHP functions in a few ways:
+Default argument values can be defined with the ``default`` filter in the
+macro body, as above, or as in native PHP functions:
 
-* Default argument values are defined by using the ``default`` filter in the
-  macro body;
+.. code-block:: jinja
+
+    {% macro input(name, value, type = 'text', size = 20) %}
+        <input type="{{ type }}" name="{{ name }}" value="{{ value|e }}" size="{{ size }}" />
+    {% endmacro %}
+
+Macros differ from native PHP functions in a few ways:
 
 * Arguments of a macro are always optional.