Add help texts to K8 form fields

What does this MR do?

Add help texts to the fields in the "Add existing k8 cluster" form that provide guidance about the values expected in them.

Note: In order to see the environment field in the "Add existing k8 cluster" form, you should test this EE-port MR.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #59275 (closed)