[ResponseOps] better documentation on proxyHeaders config

[pmuellr]

Kibana version: 8.0.0

The documentation of the action config xpack.actions.proxyHeaders is a little misleading. When looking at the code, it's only used in a few situations:

• https proxy, but not http proxy for all connectors but email
• https and http proxy for email

I don't think this is neccessarily a bug - http and https proxies are wildly different protocols, it's not clear to me there's a general way to send a proxy headers that aren't passed onto the proxied service for http.

So we should at least document the current behavior.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[pmuellr]

see also: #125837

[mikecote]

@pmuellr have we improved this area recently with another issue?

[pmuellr]

Not that I know of. But I added it to the proxy TODO issue #138319