Update healthcheck docs regarding containerPort

[sonu27]

Fixes issue #241

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[sonu27]

@nicksardo Let me know if this is clear enough :)

[nicksardo]

Thank you for creating this PR @sonu27! However, it's not matching the servicePort, it's matching the service's targetPort. It's also complicated because names can be used...

The pod's `containerPort` field must be defined.
The service's `targetPort` field must point to the pod port's `containerPort` value or `name`. Note that the `targetPort` defaults to the `port` value if not defined.

Feel free to re-phrase any of that.

This might make more sense
Ingress servicePort ▶ Service port.port || port.name
Service targetPort ▶ Pod port.containerPort || port.name
ReadinessProbe port ▶ Pod port.containerPort || port.name

[sonu27]

Your wording is good.

I was thinking of added the bottom bit, as it explains it cleanly but possibly too much :)

[nicksardo]

/lgtm