Backport of docs: document behaviour of tls.https.verify_outgoing into release/1.15.x

[dhiaayachi]

Backport

This PR is auto-generated from hashicorp#20555 to be assessed for backporting due to the inclusion of the label backport/1.15.

🚨

Warning automatic cherry-pick of commits failed. If the first commit failed,
you will see a blank no-op commit below. If at least one commit succeeded, you
will see the cherry-picked commits up to, not including, the commit where
the merge conflict occurred.

The person who merged in the original PR is:
@lkysow
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: POST https://api.github.com/repos/hashicorp/consul/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

---

At first it's not clear what verify_outgoing would do for the https listener as it seems like Consul agent's don't make https requests. Upon further investigation, it's clear that Consul agents do make https requests in the following scenarios:

• to implement watches
• to perform checks

In the first scenario, this setting is used here:
https://github.com/hashicorp/consul/blob/a1c8d4dd19caad13edf2d86441d1b7f9bbdc9c34/agent/config/runtime.go#L1725

In the second scenario, it's actually the internal_rpc setting that is used:
https://github.com/hashicorp/consul/blob/a1c8d4dd19caad13edf2d86441d1b7f9bbdc9c34/tlsutil/config.go#L903

---

Overview of commits

• ded3c9f - f6ce7c3

[dhiaayachi]

Thanks for reporting this! This issue is a known issue with GitHub's API and will be addressed as soon as possible by HashiCorp's team.