doc: PQinitOpenSSL and PQinitSSL are obsolete in OpenSSL 1.1.0+

doc: PQinitOpenSSL and PQinitSSL are obsolete in OpenSSL 1.1.0+

Starting with OpenSSL 1.1.0 there is no need to call PQinitOpenSSL
or PQinitSSL to avoid duplicate initialization of OpenSSL.  Add a
note to the documentation to explain this.

Backpatch to all supported versions as older OpenSSL versions are
equally likely to be used for all branches.

Reported-by: Sebastien Flaesch <sebastien.flaesch@4js.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/DBAP191MB12895BFFEC4B5FE0460D0F2FB0459@DBAP191MB1289.EURP191.PROD.OUTLOOK.COM
Backpatch-through: 11, all supported versions