In commit e984ef58 I (tgl) simplified the behavior of \p to just print
the current query buffer; but Daniel Vérité points out that this made it
inconsistent with the behavior of \g and \w.  It should print the same
thing \g would execute.  Fix that, and improve related comments.

Daniel Vérité

Discussion: https://postgr.es/m/9b4ea968-753f-4b5f-b46c-d7d3bf7c8f90@manitou-mail.org