Added clarification about user, client, and instance IDs.
Added clarification about user, client, and instance IDs.

As discussed here:
https://github.com/uProxy/uProxy/pull/271/files#r15001426

itplanes/uproxy,IveWong/uproxy,qida/uproxy,qida/uproxy,dhkong88/uproxy,IveWong/uproxy,itplanes/uproxy,dhkong88/uproxy,uProxy/uproxy,IveWong/uproxy,roceys/uproxy,qida/uproxy,uProxy/uproxy,chinarustin/uproxy,chinarustin/uproxy,roceys/uproxy,jpevarnek/uproxy,uProxy/uproxy,dhkong88/uproxy,MinFu/uproxy,uProxy/uproxy,roceys/uproxy,itplanes/uproxy,roceys/uproxy,IveWong/uproxy,chinarustin/uproxy,chinarustin/uproxy,MinFu/uproxy,chinarustin/uproxy,uProxy/uproxy,MinFu/uproxy,MinFu/uproxy,itplanes/uproxy,MinFu/uproxy,jpevarnek/uproxy,dhkong88/uproxy,jpevarnek/uproxy,dhkong88/uproxy,qida/uproxy,jpevarnek/uproxy,roceys/uproxy,qida/uproxy,itplanes/uproxy,jpevarnek/uproxy