Document server-to-server redirect behavior

[alanking]

We should add some detailed documentation about how iRODS works as a distributed system; specifically, how server-to-server redirects work for updating the catalog and moving data while connected to various servers of differing roles within or between iRODS zones.

I ran into some trouble with the pam_interactive plugin which could have been avoided (or at least easily solved) had I known that server-to-server connections are reused within any given agent. You can read about that story here: irods/irods_auth_plugin_pam_interactive#47

This comment gives some insight into how things are working under the hood:
https://github.com/irods/irods/blob/bc2a9bf0d8d3587d1aad8bf6a9b3db650e0d6944/plugins/api/src/switch_user.cpp#L281-L284

It may be useful for others to know this information - and in more detail - as one develops iRODS features, debugs issues in a deployment, or plans out a system and writes policy. So, we should put it in the proper documentation so it's easier to learn about it.