Create an informative issue (See each section, incomplete templates/issues won't be triaged)

Using the current documentation as a model, please complete the issue template. 

Note: Doc team updates the current version and the two previous versions (n-2). For earlier versions, we will address only high-priority, customer-reported issues for releases in support.

Prerequisite: Start with what we have

Always look at the current documentation to describe the change that is needed. Use the source or portal link for Step 4:

 - Use the Customer Portal: https://access.redhat.com/documentation/en-us/red_hat_advanced_cluster_management_for_kubernetes

 - Use the GitHub link to find the staged docs in the repository: https://github.com/stolostron/rhacm-docs 

Describe the changes in the doc and link to your dev story

Provide info for the following steps:

2. - [ ] Mandatory Choose the type of documentation change.

      - [ ] New topic in an existing section or new section
      - [x] Update to an existing topic

4. - [ ] Mandatory for bugs: What is the diff? Clearly define what the problem is, what the change is, and link to the current documentation:

Please include the following as part of Submariner pre-requisites:

In page https://github.com/stolostron/rhacm-docs/blob/2.7_stage/add-ons/submariner/subm_configure.adoc#prerequisites add the following note.

"If the gateway nodes are directly reachable over their private IPs without any NAT in between, ensure that firewall configuration allows ESP protocol on the gateway nodes."

This note is applicable for all the Submariner releases. So, please make necessary changes in the appropriate sections of the documentation.

Note: Just for reference, the following PR is used to update the upstream document - https://github.com/submariner-io/submariner-website/pull/915