Add docs for signature template

Author: leoj3n

bit-docs.js

@@ -3,9 +3,10 @@ var processJavaScript = require("./process/javascript");
 var path = require("path");
 
 /**
- * @module {function} bit-docs-js
  * @parent plugins
+ * @module {function} bit-docs-js
  * @group bit-docs-js/tags tags
+ * @group bit-docs-js/templates templates
  *
  * @description Tags, templates, and basic styles for JavaScript.
  *

templates/signature.mustache.md

@@ -0,0 +1,25 @@
+@parent bit-docs-js/templates
+@page bit-docs-js/templates/signature signature
+
+@description The function signature template.
+
+@body
+
+The `signature.mustache` template outputs a function signature.
+
+For tips on adding this template to your custom theme, see
+[bit-docs-generate-html/theme/templates/content].
+
+You can use the following snippet:
+
+```
+{{#if signatures}}
+    {{#each signatures}}
+        {{> signature.mustache}}
+    {{/each}}
+{{else}}
+    {{#types}}
+        {{> signature.mustache}}
+    {{/types}}
+{{/if}}
+```