callout.html: drop whitespace and better usage comment.

Author: XhmikosR

_includes/callout.html

@@ -1,5 +1,9 @@
-{% comment %} allowed: info danger warning {% endcomment %}
-{% assign css_class = include.type | default: "info" %}
+{%- comment -%}
+Usage: {% include callout.html content=callout type="type" %},
+where content is a capture with the content
+and type is one of: info (default), danger, warning
+{%- endcomment -%}
+{%- assign css_class = include.type | default: "info" -%}
 <div class="bd-callout bd-callout-{{ css_class }}">
-  {{ include.content | markdownify }}
+  {{- include.content | markdownify -}}
 </div>