Various psql slash commands that normally act on the current query buffer
will automatically recall and re-use the most recently executed SQL command
instead, if the current query buffer is empty.  Although this behavior is
ancient (dating apparently to commit 77a47299), it was documented nowhere
in the psql reference page.  For that matter, we'd never bothered to define
the concept of "current query buffer" explicitly.  Fix that.  Do some
wordsmithing on relevant command descriptions to improve clarity and
consistency.

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