include template controller scenarios in content projection docs

> The new Shadow DOM v1 spec has support for slot "fallback content". As a result, our replaceable attribute is no longer needed and has been removed in favor of slot fallbacks. However, slots do not solve the problem of replacing a template that is used by a repeat, if or other template controller. So, we still support placing the part attribute on a template controller's element and overriding it by using replace-part in the element's content.
> http://blog.durandal.io/2016/06/08/getting-prepared-for-aurelia-1-0-0-rc/

we need to add this somewhere in here: http://aurelia.io/hub.html#/doc/article/aurelia/templating/latest/templating-content-projection

note: `replaceable` is still required

Start with the content projection documentation at the linked Aurelia templating article and review its discussion of slot fallbacks and replaceable content. Document the template-controller scenario described in the issue, including part and replace-part, while retaining the note that replaceable is still required. Done means the relevant documentation clearly covers this exception.

Written by the indexing model from the issue text.