docs: update realm attribute description in OIDC plugin

[kayx23]

Description

Update realm description in the OIDC plugin to explain how this attribute is actually used by APISIX after checking the code. The English doc didn't do enough explanation and the Chinese doc was explaining what realm was. It was unclear why one should configure this attribute, as realm is usually already part of the discovery endpoint.

References

• https://github.com/apache/apisix/blob/master/apisix/plugins/openid-connect.lua
• https://github.com/apache/apisix/blob/master/t/plugin/openid-connect.t#L411-L419

Checklist

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