Use capitalized ID column name in get_adjacent_post(). props hypertextranch. fixes #24575.

git-svn-id: http://core.svn.wordpress.org/trunk@24506 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Correct inline docs for is_wp_error(). props johnbillion. fixes #24637.

git-svn-id: http://core.svn.wordpress.org/trunk@24507 1a063a9b-81f0-0310-95a4-ce76da25c4cd