Fix some errhint and errdetail strings missing a period

Michael Paquier <michael@paquier.xyz>

Commit: 730422afcdb6784bbe20efc65de72156d470b0c4
Author: Michael Paquier <michael@paquier.xyz>
Date: 2018-12-06T22:47:42Z
Releases: 12.0
Fix some errhint and errdetail strings missing a period

As per the error message style guide of the documentation, those should
be full sentences.

Author: Daniel Gustafsson
Reviewed-by: Michael Paquier, Álvaro Herrera
Discussion: https://1E8D49B4-16BC-4420-B4ED-58501D9E076B@yesql.se

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