Fix various typos and grammar errors in comments.

Andres Freund <andres@anarazel.de>

Commit: 6aab1f45acaa4cf90e62357ebdf5e6a38829204e
Author: Andres Freund <andres@anarazel.de>
Date: 2015-04-26T16:42:31Z
Releases: 9.5.0
Fix various typos and grammar errors in comments.

Author: Dmitriy Olshevskiy
Discussion: 553D00A6.4090205@bk.ru

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