improve docs related to using multiple profiles

(from https://github.com/GoogleContainerTools/skaffold/issues/2535#issuecomment-1539160768)

This flags some issues that users found when following steps in the [docs](https://skaffold.dev/docs/environment/profiles/#activating-multiple-profiles-at-the-same-time),

Activating multiple profiles does work, but I think the docs [need improvement](https://github.com/GoogleContainerTools/skaffold/issues/2535#8069).

The docs should make it clearer that [patches](https://skaffold.dev/docs/environment/profiles/#override-via-patches) are needed to append to lists, such as lists of helm releases or kustomize paths. This applies to profiles that overlay on the root level, as well as when multiple profiles are activated.

In addition, there is still an issue patching-over an empty value (https://github.com/GoogleContainerTools/skaffold/issues/2741, https://github.com/GoogleContainerTools/skaffold/issues/3446, https://github.com/GoogleContainerTools/skaffold/issues/8637). I suggest calling this out in the docs.

To help troubleshoot,

skaffold diagnose will show the resulting Config.

The new skaffold render will output the resulting manifests.

Also see:

Skaffold Profile doesn't allow to patch /deploy/helm/releases/0/setValues/global/namespace #6908
https://github.com/GoogleContainerTools/skaffold/issues/8379#issuecomment-1409542926