core: better synced folder plugin docs

lib/vagrant/plugin/v2/synced_folder.rb

@@ -3,15 +3,33 @@ module Vagrant
     module V2
       # This is the base class for a synced folder implementation.
       class SyncedFolder
+        # This is called early when the synced folder is set to determine
+        # if this implementation can be used for this machine. This should
+        # return true or false.
+        #
+        # @return [Boolean]
         def usable?(machine)
         end
 
+        # This is called before the machine is booted, allowing the
+        # implementation to make any machine modifications or perhaps
+        # verifications.
+        #
+        # No return value.
         def prepare(machine, folders, opts)
         end
 
+        # This is called after the machine is booted and after networks
+        # are setup.
+        #
+        # No return value.
         def enable(machine, folders, opts)
         end
 
+        # This is called after destroying the machine during a
+        # `vagrant destroy`.
+        #
+        # No return value.
         def cleanup(machine)
         end
       end