Clarify constructor return types for RNA Transcription exercise (#1695)

exercism · Jun 2, 2023 · dd0ea4e · dd0ea4e
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diff --git a/exercises/practice/rna-transcription/.docs/instructions.append.md b/exercises/practice/rna-transcription/.docs/instructions.append.md
@@ -7,3 +7,7 @@ string has a valid RNA string, we don't need to return a `Result`/`Option` from
 `into_rna`.
 
 This explains the type signatures you will see in the tests.
+
+The return types of both `DNA::new()` and `RNA::new()` are `Result<Self, usize>`,
+where the error type `usize` represents the index of the first invalid character 
+(char index, not utf8).