Revise RFC #389 with details about its argument

text/0389-dynamic-tag-names.md

@@ -43,14 +43,29 @@ With the `element` helper proposed in this RFC, this can be accomplished with so
 
 ## Detailed design
 
-We propose to add a new `element` helper that takes a tag name and generates a contextual component that, when invoked, renders the selected element.
+We propose to add a new `element` helper that takes a single positional argument.
+
+* When passed a non-empty string it will generates a contextual component that, when invoked, renders an element with the same tag name as the passed string, along with the passed attributes (if any) and yields to given block (if any).
+* When passed an empty string, it will generates a contextual compoment that, when invoked, yields to the given block without wrapping it an element and ignores any passed attributes.
+* When passed `null` or `undefined`, it will return `null`.
+* When passed any other values (e.g. a boolean or a number), it will result in a development mode assertion.
 
 Example:
 
 ```hbs
 {{#let (element @htmlTag) as |Tag|}}
-  <Tag>...</Tag>
+  <Tag class="my-element">Hello</Tag>
 {{/let}}
+
+{{!-- when @htmlTag="div" --}}
+<div class="my-element">Hello</div>
+
+{{!-- when @htmlTag="" --}}
+Hello
+
+{{!-- when @htmlTag=null or @htmlTag=undefined, it renders nothing --}}
+
+{{!-- when @htmlTag=true or @htmlTag=1, it throws in development mode --}}
 ```
 
 Unlike ids, classes or other attributes, the tag name of DOM element cannot be changed in runtime.