Docs on #post context don't mention visibility attribute

Issue Summary
The documentation of the Post object at Post Context: Ghost Themes - Documentation doesn’t include the visibility attribute.

I actually didn’t think think visibility was accessible until I found these forum topics via search:

You can see from these topics that there is confusion among theme authors about the exact values this attribute can have.

Thanks.

Members related documentation is in the members docs with a section that covers access and content visibility https://ghost.org/docs/themes/members/#content-visibility

We can definitely look at adding a link to that from the post context section though. Thanks for raising it.

Ah, thanks for the pointer.

I think it would be a great idea to link to the content visibility page from the Post Context one.