Document additional envvars that support appending _identifier

[feorlen]

Some environment variables allow appending a custom identifier to set multiple values. Example:

Object Lambda

export MINIO_LAMBDA_WEBHOOK_ENABLE_myfunction="on"
export MINIO_LAMBDA_WEBHOOK_ENDPOINT_myfunction="http://webhook-1.example.net"
export MINIO_LAMBDA_WEBHOOK_ENABLE_yourfunction="on"
export MINIO_LAMBDA_WEBHOOK_ENDPOINT_yourfunction="http://webhook-2.example.net"

Other envvars support this as well, per a recent customer discussion:

MINIO_IDENTITY_OPENID_DISPLAY_NAME_something="Different OIDC"

Goals

• Document which envvars can use _identifier to have multiple values.
• If all/most, add something in the MinIO Server envvars section describing this.

[feorlen]

More examples

         set MINIO_AUDIT_WEBHOOK_ENABLE_<IDENTIFIER>="on"
         set MINIO_AUDIT_WEBHOOK_ENDPOINT_<IDENTIFIER>="https://webhook-1.example.net"
         set MINIO_AUDIT_WEBHOOK_AUTH_TOKEN_<IDENTIFIER>="TOKEN"
         set MINIO_AUDIT_WEBHOOK_CLIENT_CERT_<IDENTIFIER>="cert.pem"
         set MINIO_AUDIT_WEBHOOK_CLIENT_KEY_<IDENTIFIER>="cert.key"