more plugin doc updates (#7767)

plugin/api.go

@@ -7,8 +7,10 @@ import (
 	"github.com/mattermost/mattermost-server/model"
 )
 
-// API implementations can be used to retrieve data or perform actions on behalf of the plugin. Most
-// methods have direct counterparts in the REST API and very similar behavior.
+// The API can be used to retrieve data or perform actions on behalf of the plugin. Most methods
+// have direct counterparts in the REST API and very similar behavior.
+//
+// Plugins can obtain access to the API by implementing the OnActivate hook.
 type API interface {
 	// LoadPluginConfiguration loads the plugin's configuration. dest should be a pointer to a
 	// struct that the configuration JSON can be unmarshalled to.