Add documentation of GraphQL API copy_field_description helper

doc/development/api_graphql_styleguide.md

@@ -470,6 +470,23 @@ field :confidential, GraphQL::BOOLEAN_TYPE, description: 'Indicates the issue is
 field :closed_at, Types::TimeType, description: 'Timestamp of when the issue was closed'
 ```
 
+### `copy_field_description` helper
+
+Sometimes we want to ensure that two descriptions will always be identical.
+For example, to keep a type field description the same as a mutation argument
+when they both represent the same property.
+
+Instead of supplying a description, we can use the `copy_field_description` helper,
+passing it the type, and field name to copy the description of.
+
+Example:
+
+```ruby
+argument :title, GraphQL::STRING_TYPE,
+          required: false,
+          description: copy_field_description(Types::MergeRequestType, :title)
+```
+
 ## Authorization
 
 Authorizations can be applied to both types and fields using the same