Update notes about behavior

text/0000-promote-in-element-to-public-api.md

@@ -23,10 +23,13 @@ time to make it public.
 # Detailed design
 
 The existing API of `{{-in-element}}` is very simple and I do not suggest making any change to it.
-It takes a DOM element as the only positional param and a block, and renders that block _inside_ the
-given element instead of where it would normally go.
 
-P.e.
+* It takes a single positional param `destinationElement` that is a DOM element, and a block
+* The given block is rendered not where it is located, but inside the given `destination` element.
+* If `destinationElement` is false/null/undefined then it doesn't render anything but it doesn't error.
+* If `destinationElement` changes the block is removed from the previous destination and added to the new one.
+
+Example usage:
 
 ```hbs
 {{#-in-element destinationElement}}