Fix HA docs API access endpoints explained (#2126)

* Fix HA docs API access endpoints explained

Follow-up commit 81347298a3ef7932cbeb55e877644ca22d1625f1
and fix the endpoint value provided in HA docs.

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>

* Clarify internal LB with external LB use case

* Clarify how to use both internal and external, non-cluster aware and
  not managed with Kubespray, LB solutions.
* Clarify the requirements, like TLS/SSL termination, for such an external LB.
  Unlike to the 'cluster-aware' external LB config, endpoints' security must be
  managed by that non-cluster aware external LB.
* Note that masters always contact their local apiservers via https://bip:sp.
  It's highly unlikely to go down and it reduces latency that might be
  introduced when going host->lb->host. Only computes go that path.

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>

* Add a note for supplementary_addresses_in_ssl_keys

Explain how to benefit from supplementary_addresses_in_ssl_keys

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>