Return 404 when querying author's posts who is not a member and has no posts on the site

fixes #20601. props yoavf, nacin, SergeyBiryukov, wonderboymusic, markjaquith.

git-svn-id: https://develop.svn.wordpress.org/trunk@27290 602fd350-edb4-49c9-b593-d223f7449a82
Add inline documentation for properties and other inline docs fixes for `WP_Adjacent_Post`.

See #26937.


git-svn-id: https://develop.svn.wordpress.org/trunk@27291 602fd350-edb4-49c9-b593-d223f7449a82
Remove now-moot vanity spacing for `WP_Adjacent_Post` property definitions.

See #26937.


git-svn-id: https://develop.svn.wordpress.org/trunk@27292 602fd350-edb4-49c9-b593-d223f7449a82