improve documentation of html-custom-element

doc/html-custom-element.md

@@ -10,10 +10,10 @@ provides some additional, often used methods.
 equivalent of native getElementById, but works asynchroneous and
 returns a Promise which resolves when the requested element is
 aviable.
-*Parameters*
+**Parameters**
 - id {string} - id of the requested element
 
-*Return*
+**Return**
 - {Promise<Node>} - resolve contains reference to the requested element
 
 ### addAttributeListener(Node, attrName, fn, IMT)
@@ -22,5 +22,5 @@ like an event listener but for attributes. It observes an element
 attribute for changes and invokes a pre defined callback function. For
 this js MutationObservers are used.
 
-*Parameters*
--
+**Parameters**
+- Node {Node}: reference to the element