I18n: Begin introducing translator comments for strings which include...

I18n: Begin introducing translator comments for strings which include placeholders but no accompanying translator comment.

Adds context to one string used in two different contexts for the new user and new site signup email notification.

More to come.

See #38882

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git-svn-id: https://core.svn.wordpress.org/trunk@39263 1a063a9b-81f0-0310-95a4-ce76da25c4cd