ip-restriction plugin support DIY message

[belongwqz]

Improve Docs

Please describe which part of docs should be improved or typo fixed

ip-restriction plugin support DIY message

Describe the solution you'd like

current message is a hard code: Your IP address is not allowed.
if we can DIY, it should be better.

[tokers]

@belongwqz PR's welcome. BTW, it's not a doc-related issue.