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