doc: Avoid writing in first-person form

doc: Avoid writing in first-person form

This rewrites the few places where first-person form was used, and
replaces with wording more in line with the rest of the documentation.

While there, the section on editing PO files is split between what
to edit with, and how to edit, as well as add a missing colon.

Author: Daniel Gustafsson <daniel@yesql.se>
Reported-by: wolakk@gmail.com
Discussion: https://postgr.es/m/166692508416.630.9910522387468316027@wrigleys.postgresql.org