Improve WritebackContextInit() comment and prototype argument names.

Andres Freund <andres@anarazel.de>

Commit: 48bfeb244ff280bad9a81c67442065ae8fcda6b8
Author: Andres Freund <andres@anarazel.de>
Date: 2016-07-01T21:29:03Z
Releases: 9.6.0
Improve WritebackContextInit() comment and prototype argument names.

Author: Masahiko Sawada
Discussion: CAD21AoBD=Of1OzL90Xx4Q-3j=-2q7=S87cs75HfutE=eCday2w@mail.gmail.com

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