Add user documentation for iRODS 4.3 upgrade

[mikkonie]

There needs to be user-facing documentation regarding the iRODS 4.3 upgrade in the manual. This probably should go under the irodsinfo document, but I'll see what the most appropriate place is. Also there should be links to this from "migration guide" and whereever appropriate.

Things to Document

• If connecting from your own workstation or managing a server, upgrade icommands to the latest version
  • 4.3.3 at the time of writing
• Retrieve a new irods_environment.json file from irodsinfo and replace it with your own, OR edit your existing file
  • "irods_authentication_scheme": "PAM" -> "irods_authentication_scheme": "pam_password"
  • TODO: Other changes required?
• TBD: Other things to document?