* updated obsoleted RFCs to current versions
* linked most references to their RFCs
* linked extension commands to their RFCs
* removed unidiomatic `()` from instance method links
* escaped `IMAP` in a few places
* converted all response structs to explicit classes: this makes much
nicer rdoc output than listing them all under "constants"
* grouped flags constants into their own sections
https://github.com/ruby/net-imap/commit/9cd562ac84