Add note in BMCS builder docs re subnet comm port

Communicator must be able to communicate with instance via configured
subnet. The default communicator and default BMCS subnet work out of
the box, however, this makes required config for custom set-ups
clearer. Resolves @dave2's comment on
https://github.com/mitchellh/packer/pull/4554.

website/source/docs/builders/oracle-bmcs.html.md

@@ -110,6 +110,10 @@ builder.
     [ListSubnets](https://docs.us-phoenix-1.oraclecloud.com/api/#/en/iaas/latest/Subnet/ListSubnets)
     operation available in the Core Services API.
 
+    Note: the subnet must be configured to allow access via your chosen
+    [communicator](/docs/templates/communicator.html) (communicator defaults to
+    [SSH tcp/22](/docs/templates/communicator.html#ssh_port)).
+
  -  `tenancy_ocid` (string) - The OCID of your tenancy. Overrides value provided
     by the
     [BMCS config file](https://docs.us-phoenix-1.oraclecloud.com/Content/API/Concepts/sdkconfig.htm)