Fix rewrite documentation typo and clarify with "rules array" over "this/the...

Fix rewrite documentation typo and clarify with "rules array" over "this/the structure". props SergeyBiryukov. see #16687


git-svn-id: https://core.svn.wordpress.org/trunk@18566 1a063a9b-81f0-0310-95a4-ce76da25c4cd