Updates to connecting-to-a-portal

Tackled a few doc issues:

[x] added auth_user() per https://github.com/R-ArcGIS/planning/issues/1 (this included a bit of restructuring of heading levels)
[x] clarified need to restart R session for env variables (and Pro active portal change) per https://github.com/R-ArcGIS/planning/issues/1
[x] updated set_auth_token() references to set_arc_token()
[x] expanded auth_binding() section
[x] other general edits, including adding a note with links to more info about the ArcGIS sharing model, since that topic is fairly complex

R-ArcGIS / r-bridge-site