Document Scope#$destroy requirement in $transclude

[IgorMinar]

see #9281 (comment)

[petebacondarwin]

@IgorMinar - if a directive is not manually modifying the DOM (i.e. only using built-in directives to add and remove DOM elements) then there should be no need for the application developer to worry or even do anything special in their directives. It is only in the case where their directive takes responsibility for DOM manipulation where they have to ensure that they tidy up their DOM and scopes.

I will find some place to put this info...