Document commands that do not work with 1.2 shell

Some commands do not work from 1.2 shell when running against 2.0 server. Add a section to the reference guide mentioning the incompatibilities.

Some of these are collected in this document: https://docs.google.com/document/d/1l2ad5G_GUk0WQ02xKeKO6mTbpdyZjU1NGuPPI42Wbf4/edit