namesys/interface: Expand package docs to discuss mutable names

What they are, why you'd use them, and which command-line tools you
can use to access this functionality.

namesys/interface.go

-// package namesys implements various functionality for the ipns naming system.
+/*
+Package namesys implements resolvers and publishers for the IPFS
+naming system (IPNS).
+
+The core of IPFS is an immutable, content-addressable Merkle graph.
+That works well for many use cases, but doesn't allow you to answer
+questions like "what is Alice's current homepage?".  The mutable name
+system allows Alice to publish information like:
+
+  The current homepage for alice.example.com is
+  /ipfs/Qmcqtw8FfrVSBaRmbWwHxt3AuySBhJLcvmFYi3Lbc4xnwj
+
+or:
+
+  The current homepage for node
+  QmatmE9msSfkKxoffpHwNLNKgwZG8eT9Bud6YoPab52vpy
+  is
+  /ipfs/Qmcqtw8FfrVSBaRmbWwHxt3AuySBhJLcvmFYi3Lbc4xnwj
+
+The mutable name system also allows users to resolve those references
+to find the immutable IPFS object currently referenced by a given
+mutable name.
+
+For command-line bindings to this functionality, see:
+
+  ipfs name
+  ipfs dns
+  ipfs resolve
+*/
 package namesys
 
 import (