Document pitfalls for private changes and discourage using them for security fixes
Document pitfalls for private changes and discourage using them for security fixes

Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>
Change-Id: I2a71b66a3f4b50aaa73b415f46de4f4296d4389b

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