pgsql: Document psql's behavior of recalling the previously executedqu

Document psql's behavior of recalling the previously executed query.

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 77a472993), 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

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/68dba97a4dea5c5c915e31978a475107c17c458d

Modified Files
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doc/src/sgml/ref/psql-ref.sgml | 73 ++++++++++++++++++++++++++++++------------
1 file changed, 52 insertions(+), 21 deletions(-)