MDL-21695 Admin setting descriptions are expected in Markdown syntax now

This is backwards compatible as the strings actually do not use
formatting at all. Using Markdown syntax allows us to use the same
string as we use for help.

lib/adminlib.php

@@ -1210,7 +1210,7 @@ abstract class admin_setting {
     public $name;
     /** @var string localised name */
     public $visiblename;
-    /** @var string localised long description */
+    /** @var string localised long description in Markdown format */
     public $description;
     /** @var mixed Can be string or array of string */
     public $defaultsetting;
@@ -1232,7 +1232,7 @@ abstract class admin_setting {
     public function __construct($name, $visiblename, $description, $defaultsetting) {
         $this->parse_setting_name($name);
         $this->visiblename    = $visiblename;
-        $this->description    = $description;
+        $this->description    = markdown_to_html($description);
         $this->defaultsetting = $defaultsetting;
     }