Update file kubernetes.md
What does this MR do?
Adds a note about pod_spec overwriting configuration values set in [runners.kubernetes] in config.toml
Why was this MR needed?
It's not currently clear from the documentation whether configurations set in [runners.kubernetes] or pod_spec will take priority in the final pod sent to the API server, so this aims to make that clearer.
What's the best way to test this MR?
It's a documentation change, so no test is necessary.
What are the relevant issue numbers?
See #36290 (comment 1516143865)
CC @ratchade