Define function composition for :string values

Author: eemeli

spec/registry.md

@@ -359,6 +359,14 @@ the `:string` selector performs as described below.
 
 The `:string` function returns the string value of the resolved value of the _operand_.
 
+#### Composition
+
+When an _operand_ or an _option_ value uses a _variable_ with a _declaration_
+using an _expression_ with a `:string` _annotation_,
+its resolved value contains only the string value of the _operand_ of the annotated _expression_,
+together with its resolved locale and directionality,
+and no _option_ values.
+
 ## Numeric Value Selection and Formatting
 
 ### The `:number` function