Add note about remote mailboxes to `$spoolfile` docs

It seems from a discussion on the mutt-users list that there was some confusion about the name of this variable and what it can refer to. Add a note explicitly clarifying that $spoolfile can refer to a remote path as well as to an actual "spool" "file".

Note: my formatting may be off - let me know if the wrap or codes look wrong here.

I don't think any of the other major high level docs about IMAP setup are in the codebase, but if they are, happy to adjust those too.