Clearly state that connectionSecretKeys is immutable

[plumbis]

There is confusion around connectionSecretKeys. There are two spots where it's discussed today:
https://docs.crossplane.io/v1.12/concepts/composition/#hl-6-22
https://docs.crossplane.io/v1.12/concepts/composition/#composite-resource-connection-secrets

Make sure to clarify that this is immutable in the Composite Resources update.

[aroramrinaal]

hey @plumbis ,
Can I work on this issue?

[plumbis]

Thanks for volunteering @aroramrinaal! For this issue, it's going to be fixed by a much larger rewrite of the entire Composition chapter, aiming to fix all the concepts/composition issues.