more plugin doc updates (#7767)

plugin/supervisor.go

@@ -3,7 +3,8 @@
 
 package plugin
 
-// Supervisor provides the interface for an object that controls the execution of a plugin.
+// Supervisor provides the interface for an object that controls the execution of a plugin. This
+// type is only relevant to the server, and isn't used by the plugins themselves.
 type Supervisor interface {
 	Start() error
 	Stop() error