chore(doc-gen): render void attributes in directive usage

When a directive doesn't take a value, its name is not included
in the parameters, which previously meant that the directive name
was missing from the Usage section of the docs.

This commit adds the name to the Usage section when it is missing
from the parameters.

Closes #14045

Author: Narretz

docs/config/templates/ngdoc/api/directive.template.html

@@ -29,11 +29,22 @@ <h2 id="usage">Usage</h2>
       </li>
     {% endif -%}
 
+
+    {% set hasNameAsParam = false %}
+
+    {# when a directive's name is not a parameter (i.e. doesn't take a value),
+      add the directive name to the list of attributes and/or css classes #}
+
+    {%- for param in doc.params %}
+    {% set hasNameAsParam = true if param.name === doc.name else hasNameAsParam %}
+    {%- endfor %}
+
     {%- if doc.restrict.attribute -%}
       <li>as attribute:
         {% code %}
-        <{$ doc.element $}
-          {%- for param in doc.params %}
+        <{$ doc.element $} {%- if not hasNameAsParam %}
+          {$ lib.directiveParam(doc.name, {}, '', '') $}
+          {%- endif -%} {%- for param in doc.params %}
           {$ lib.directiveParam(param.name, param.type, '="', '"') $}
           {%- endfor %}>
         ...
@@ -43,10 +54,14 @@ <h2 id="usage">Usage</h2>
     {% endif -%}
 
     {%- if doc.restrict.cssClass -%}
+
       <li>as CSS class:
         {% code %}
         {% set sep = joiner(' ') %}
         <{$ doc.element $} class="
+        {%- if not hasNameAsParam -%}
+          {$ sep() $}{$ lib.directiveParam(doc.name, {}, '', '') $}
+        {%- endif -%}
         {%- for param in doc.params -%}
           {$ sep() $}{$ lib.directiveParam(param.name, param.type, ': ', ';') $}
         {%- endfor %}"> ... </{$ doc.element $}>