Add documentation for embed function

[theotherdon]

Adds documentation for passing a function to embed to selectively include relationships. Looking at the diff, I see that there's a small blurb about embed receiving a function, but it's not showing up in the docs for some reason. I also elaborated on the documentation with an example. Here's a screenshot of what the docs currently look like for the embed property (note the lack of the line that mentions embed being able to receive a function):

[theotherdon]

@mansona Any chance you can give this a review for me? 😄

[theotherdon]

@osi-jehrlich Think you can take a look at this for me?