pgsql: Doc: remove misleading info about ecpg's CONNECT/DISCONNECT DEFA

Doc: remove misleading info about ecpg's CONNECT/DISCONNECT DEFAULT.

As far as I can see, ecpg has no notion of a "default" open
connection. You can do "CONNECT TO DEFAULT" but that just specifies
letting libpq use all its default connection parameters --- the
resulting connection is not special subsequently. In particular,
SET CONNECTION = DEFAULT and DISCONNECT DEFAULT simply act on a
connection named DEFAULT, if you've made one; they do not have
special lookup rules. But the documentation of these commands
makes it look like they do.

Simplest fix, I think, is just to remove the paras suggesting that
DEFAULT is special here.

Also, SET CONNECTION *does* have one special lookup rule, which
is that it recognizes CURRENT as an alias for the currently selected
connection. SET CONNECTION = CURRENT is a no-op, so it's pretty
useless, but nonetheless it does something different from selecting
a connection by name; so we'd better document it.

Per report from Sylvain Frandaz. Back-patch to all supported
versions.

Discussion: https://postgr.es/m/169824721149.1769274.1553568436817652238@wrigleys.postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c45a45fedbcffecfcd318a0a9349d3e1bbfb5e49

Modified Files
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doc/src/sgml/ecpg.sgml | 22 ++--------------------
1 file changed, 2 insertions(+), 20 deletions(-)