Define function composition for :string values (#798)

* Define function composition for :string values * Update spec/registry.md as suggested by @stasm in #814 * Drop the "only" * Update text following code review comments --------- Co-authored-by: Addison Phillips <[email protected]>
unicode-org · Oct 21, 2024 · 1291773 · 1291773
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@@ -86,6 +86,14 @@ The `:string` function returns the string value of the _resolved value_ of the _
 > Users SHOULD encode _messages_ and their parts in Unicode Normalization Form C (NFC)
 > unless there is a very good reason not to.
 
+#### Composition
+
+When an _operand_ or an _option_ value uses a _variable_ annotated,
+directly or indirectly, by a `:string` _function_,
+its _resolved value_ contains the string value of the _operand_ of the annotated _expression_,
+together with its resolved locale and directionality.
+None of the _options_ set on the _expression_ are part of the _resolved value_.
+
 ## Numeric Value Selection and Formatting
 
 ### The `:number` function