Add documentation URL for XING (#3636)
Add documentation URL for XING (#3636)

The proper documentation for the XING 2FA settings was missing and I added them.
jdavis/twofactorauth,vgarmash/twofactorauth,jdavis/twofactorauth,Jawshy/twofactorauth,Jawshy/twofactorauth,Jawshy/twofactorauth,Nitrokey/dongleauth,bjorno/twofactorauth,rj175/twofactorauth,Nitrokey/dongleauth,vgarmash/twofactorauth,Jawshy/twofactorauth,bjorno/twofactorauth,Nitrokey/dongleauth,rj175/twofactorauth,vgarmash/twofactorauth,bjorno/twofactorauth,rj175/twofactorauth,jdavis/twofactorauth