Use PQfreemem() consistently, and document its use for Notify.

doc/src/sgml/libpq.sgml

src/bin/psql/common.c

src/interfaces/ecpg/ecpglib/execute.c

src/interfaces/libpgtcl/pgtclId.c

src/interfaces/libpq/fe-exec.c

src/interfaces/libpq/libpq-fe.h

src/interfaces/python/pgmodule.c

src/test/examples/testlibpq2.c