[Doc] describe visibility of functions in more detail

[rozek]

As briefly mentioned in the overview "Functions defined on elements will be available to the element the function is defined on, as well as any child elements." (see this live example)

In my opinion this visibility is a really important detail - but could easily be overlooked in the amount of other information in this document.

For that reason, I'd suggest to give that detail more room - both in the overview and the "def" feature reference