[DOCS] Clarify certgen deprecation (elastic/x-pack-elasticsearch#4099)
[DOCS] Clarify certgen deprecation (elastic/x-pack-elasticsearch#4099)

Original commit: elastic/x-pack-elasticsearch@819f0ecf082fb41c5dd921fb335280bd8c93fd34

vroyer/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra