Document helm resource upgrade process(es)

[gjenkins8]

In the FAQ, there is a detailed description of how helm 3 does the three-way merge + patch process for upgrades:

https://helm.sh/docs/faq/changes_since_helm2/#improved-upgrade-strategy-3-way-strategic-merge-patches

But I don't think there is a detailed description in the main docs. Neither I think a very good description on how the helm upgrade --force flag changes the process (force flag impl: helm/helm#1958)

During helm issue triage, Ive noticed I've explained these things a few times. Perhaps it would be a good idea to create a doc section on how helm manages upgrades here?

(Note: helm/helm#11950 is an example of explain the issue, and the immediate inspiration for creating this)

@joejulian