docs: [batch] refine proto comment for run_as_non_root (#10404)

* docs: refine proto comment for run_as_non_root
docs: add caution messages for container runnable username and password fields

PiperOrigin-RevId: 608240389

Source-Link: googleapis/googleapis@6f599f0

Source-Link: https://github.com/googleapis/googleapis-gen/commit/6af0d38d953e7a5f9427d650ebcd3726fe286dce
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