SE-0498: demangle not prevents returning truncated unicode scalars

proposals/0498-runtime-demangle.md

@@ -47,15 +47,15 @@ public func demangle(
 public enum DemanglingResult: Equatable {
   case success
   case failed
-  case truncated(Int)
+  case truncated(required: Int)
 }
 ```
 
 The span accepting API is necessary for performance sensitive use-cases, which attempt to demangle symbols in process, before displaying or sending them for further processing. In those use-cases it is common to have a known maximum buffer size into which we are willing to write the demangled representation.
 
 The output from this API is an `OutputSpan` of `UTF8.CodeUnit`s, and it may not necessarily be well-formed UTF8, because of the potential of truncation happening between two code units which would render the UTF8 invalid.
 
-If the demangled representation does not fit the preallocated buffer, the demangle method will return `truncated(actualSize)` such that developers can determine by how much the buffer might need to be increased to handle the complete demangling. When `.truncated` is returned, the `OutputSpan` will contain a _partial result_, of however many characters were able to fit into it before truncation ocurred. This also means that a truncated output may not be entirely valid UTF8. 
+If the demangled representation does not fit the preallocated buffer, the demangle method will return `truncated(requiredSize)` such that developers can determine by how much the buffer might need to be increased to handle the complete demangling. When `.truncated` is returned, the `OutputSpan` will contain a _partial result_, of however many characters were able to fit into it before truncation ocurred. If truncation ocurrs, the returned string will contain only valid UTF8, i.e. the result will never be truncated in the middle of a unicode scalar.
 
 Converting the outputSpan to a `String`, which is guarateed to be valid UTF8, follows the below pattern, where creating an `UTF8Span` _will_ perform validation of the UTF8 String contents, and fail if the output wasn't correct.