doc: add the missing serveer CA mode CUSTOMER_MANAGED_CAS_CA.

[feng-zhe]

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[github-actions]

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
google-beta provider: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))

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