Fix incorrect return type doc

You can see it was incorrect by looking at RedirectPlugin::beforeSendPerformed
and at the implementations of the Swift_Mime_MimeEntity interface
minor #511 Fix incorrect return type doc (JeroenDeDauw)

This PR was merged into the 5.2-dev branch.

Discussion
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Fix incorrect return type doc

You can see it was incorrect by looking at RedirectPlugin::beforeSendPerformed
and at the implementations of the Swift_Mime_MimeEntity interface

Commits
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6f7b3f7 Fix incorrect return type doc