Credits: After [26354], replace another instance of the "Contributing to...

Credits: After [26354], replace another instance of the "Contributing to WordPress" Codex link with http://make.wordpress.org/.

Remove redundant translator comments, add comments for placeholders instead.

See #25806.
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git-svn-id: https://core.svn.wordpress.org/trunk@35862 1a063a9b-81f0-0310-95a4-ce76da25c4cd