JSON API: post lists and comment lists should use the linked_user_id in the author filter

For these endpoints:

/sites/$site/posts
/sites/$site/comments

We allow an `author` parameter to filter to posts or comments by that author. Given that posts and comments are supposed to include author data from the linked wpcom user (not working currently, see #1410), the ID we accept for `author` should really be the `linked_user_id`, or the wpcom User ID. Or if we want to support queries using either a jetpack User ID or the user's linked_external_id, we should have two filter parameters to support either option.

We should also make the documentation a little more clear about what this `author` parameter is expecting.

Start with the /sites/$site/posts and /sites/$site/comments endpoints and read how their author parameter is documented. Compare the linked-user behavior described in #1410, then resolve whether author should use linked_user_id or whether separate filters are needed. Done means both endpoints filter consistently and the documentation clearly states the accepted identifier.

Written by the indexing model from the issue text.