🔒 📚 Improvements and docs for SASL::ClientAdapter

[nevans]

Several minor updates, all related to the SASL::ClientAdapter API:

• Improved SASL::ClientAdapter documentation.
• Added documention to SASL::ProtocolAdapters and each of the methods in SASL::ProtocolAdapters::Generic.
• #host and #port didn't make sense in SASL::ProtocolAdapters. Moved them to SASL::ClientAdapter.
• #service returns the generic GSSAPI service name ("host") rather than raise an exception.
• Use Forwardable: almost all of the methods simply delegate to #client.
• Delete IMAP::SASLAdapter#auth_capable?: the superclass definition is identical.