transfer: small doc update
Tweak the README a little. Name and spell out axfr/ixfr.
## Name
-*transfer* - answer zone transfers requests for compatible authoritative
-plugins.
+*transfer* - perform zone transfers for other plugins.
## Description
This plugin answers zone transfers for authoritative plugins that implement
`transfer.Transferer`.
-Transfer answers AXFR requests and IXFR requests with AXFR fallback if the
-zone has changed.
+Transfer answers full zone transfer (AXFR) requests and incremental zone transfer (IXFR) requests
+with AXFR fallback if the zone has changed.
Notifies are not currently supported.
