Update Bamboo Specs document for Plan permission

Issue Summary

This is reproducible on the Data Center: yes

Plan permission in the case of multiple build plans using specs doesn't work if the permission is declared in the individual specs files. BAM-21510

Please update the documentation to either append the plan-permissions to the main bamboo.yml or to place the plan-permissions into a separate file and include it.
https://docs.atlassian.com/bamboo-specs-docs/9.5.2/specs.html?yaml#plan-permissions

Steps to Reproduce

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Expected Results

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Actual Results

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Workaround

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