Turn most mentions of libpq functions into links.  At id attributes to
most libpq functions, where not existing yet, so that they can be
linked to.  (In a handful of cases there were problems with the PDF
processing toolchain, so those instances were not changed.)

Author: Fabien COELHO <coelho@cri.ensmp.fr>
Reviewed-by: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/alpine.DEB.2.21.1905121032330.27203@lancre