Twenty Fifteen: documentation for the twentyfifteen_credits action

Props iamtakashi, fixes #30051


Built from https://develop.svn.wordpress.org/trunk@29998


git-svn-id: http://core.svn.wordpress.org/trunk@29738 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29739. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29739 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29740. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29740 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29741. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29741 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29742. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29742 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29743. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29743 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29744. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29744 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29745. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29745 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29746. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29746 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29747. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29747 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29748. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29748 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29749. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29749 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29750. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29750 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29751. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29751 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29752. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29752 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29753. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29753 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29754. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29754 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29755. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29755 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29756. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29756 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-alpha-29757. (Bumping to permanently sync the revision number with https://develop.svn.wordpress.org.)

git-svn-id: http://core.svn.wordpress.org/trunk@29757 1a063a9b-81f0-0310-95a4-ce76da25c4cd