Docs: Specify job timeout override rules

What does this MR do?

We have received feedback from users that the documentation does not specifies the rule when the job timeout overrides project timeout and what how it behaves with Runner specific timeout.

This MR fixes that.

Related issue: #14887 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines