Doc: remove misleading claim in documentation of PQreset().

Doc: remove misleading claim in documentation of PQreset().

This text claimed that the reconnection would occur "to the same
server", but there is no such guarantee in the code, nor would
insisting on that be an improvement.

Back-patch to v10 where multi-host connection strings were added.

Discussion: https://postgr.es/m/1095901.1611268376@sss.pgh.pa.us