doc: clarify about name in toml

doc/METADATA.ini

@@ -155,11 +155,16 @@ example=
 
 [internal/<plugin>/*]
 status= implemented
-usedby/plugin= (ini)
-description= Internal metadata to be ignored by other plugins.
-example=internal/ini/parent could be the name for internal parent
-       information of the ini plugin.
-
+usedby/plugin=
+description= Internal metadata that must be ignored by other plugins.
+	It is useful for reconstruction of information about keys between
+	kdbGet and kdbSet.
+	Always avoid to use it if there is any other metadata that
+	can be used instead.
+	For example, if a type can be represented with the CORBA
+	type system, metadata `type` should be used.
+	Only if a type cannot be represented, e.g., the `toml`
+	plugin would use `internal/toml/type`.
 
 [line]
 type= int