Describe the usage of global ID types in the style-guide

doc/development/api_graphql_styleguide.md

@@ -49,6 +49,20 @@ See also:
 - [Exposing Global IDs](#exposing-global-ids).
 - [Mutation arguments](#object-identifier-arguments).
 
+We have a custom scalar type (`Types::GlobalIDType`) which should be used as the
+type of input and output arguments when the value is a `GlobalID`. The benefits
+of using this type instead of `ID` are:
+
+- it validates that the value is a `GlobalID`
+- it parses it into a `GlobalID` before passing it to user code
+- it can be parameterized on the type of the object (e.g.
+  `GlobalIDType[Project]`) which offers even better validation and security.
+  
+Consider using this type for all new arguments and result types. Remember that
+it is perfectly possible to parameterize this type with a concern or a
+supertype, if you want to accept a wider range of objects (e.g.
+`GlobalIDType[Issuable]` vs `GlobalIDType[Issue]`).
+
 ## Types
 
 We use a code-first schema, and we declare what type everything is in Ruby.