Rephrase comments to make them clearer

Rephrase comments to make them clearer

The use of "we" when referring to the active backend might be
misunderstood, so rephrase to make it clearer who is performing
the actions discussed in the comment.

Author: Junwang Zhao <zhjwpku@gmail.com>
Reviewed-by: Erikjan Rijkers <er@xs4all.nl>
Reviewed-by: Robert Treat <rob@xzilla.net>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAEG8a3LRSMqkvjiURiJoSi4aGWORpiXUmUfQQK5PaD6WfPzu3w@mail.gmail.com