Godoc fixes (#898)

Add some godoc comments for the api/agent package and some of its
subpackages.

api/agent/doc.go

@@ -0,0 +1,26 @@
+// Package agent defines the Agent interface and related concepts. An agent is
+// an entity that knows how to execute an Fn function.
+//
+// The Agent Interface
+//
+// The Agent interface is the heart of this package. Agent exposes an api to
+// create calls from various parameters and then execute those calls. An Agent
+// has a few roles:
+//	* manage the memory pool for a given server
+//	* manage the container lifecycle for calls (hot+cold)
+//	* execute calls against containers
+//	* invoke Start and End for each call appropriately
+//	* check the mq for any async calls, and submit them
+//
+// For more information about how an agent executes a call see the
+// documentation on the Agent interface.
+//
+// Variants
+//
+// There are two flavors of runner, the local Docker agent and a load-balancing
+// agent. To create an agent that uses Docker containers to execute calls, use
+// New().
+//
+// To create an agent that can load-balance across a pool of sub-agents, use
+// NewLBAgent().
+package agent