docs: Supplements the openid-connect configuration doc

[wensenz]

This configuration is used when token introspection endpoints require additional parameters

Description

Supplements the openid-connect configuration document

Checklist

• [x] I have explained the need for this PR and the problem it solves
• [x] I have explained the changes or the new features added to this PR
• [x] I have added tests corresponding to this change
• [x] I have updated the documentation to reflect this change
• [x] I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)