Document several metamethods

Author: uzluisf

doc/Type/Metamodel/PrivateMethodContainer.pod6

@@ -42,11 +42,23 @@ Adds a private method C<$code> with name C<$name>.
 
 Returns a hash of C«name => &method_object»
 
-=begin comment
+=head2 method private_methods
 
-TODO: document find_private_method, once we've figured out how to represent
-or catch nqp::null in Raku land
+    method private_methods($obj)
 
-=end comment
+Returns a list of private method names.
+
+=head2 method private_method_names
+
+    method private_method_names($obj)
+
+Alias to C<private_methods>.
+
+=head2 method find_private_method
+
+    method find_private_method($obj, $name)
+
+Locates a private method. Otherwise, returns L<C<Mu>|/type/Mu> if it doesn't
+exist.
 
 =end pod