Update Security doc for 1.10.* (in which we have two Web UIs)

[XD-DENG]

This PR is to address out-dated documentation issue in 1.10., in which we still have two sets of web UI supported. The stable doc of 1.10. is not reflecting the authentication feature of FAB-based UI properly.

Note this PR is raised against V1-1-stable

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Issue link: WILL BE INSERTED BY boring-cyborg

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[XD-DENG]

Hi @kaxil and @potiuk , please take a look.

Please let me know if you prefer this to be raised against V1-10-test instead.

Thanks!