SF-0033: Clarify which "case-insensitivity" is used.

In response to: https://forums.swift.org/t/review-sf-0033-string-encoding-names/81965/8

Author: YOCKOW

Proposals/0033-String-Encoding-Names.md

@@ -224,7 +224,7 @@ Accordingly, this proposal suggests just simple correspondence between `String.E
 
 ### Name to `String.Encoding`
 
-- `init(ianaName:)` adopts case-insensitive comparison with *Preferred MIME Name*, *Name*, and *Aliases*.
+- `init(ianaName:)` adopts ASCII case-insensitive comparison with *Preferred MIME Name*, *Name*, and *Aliases*.
 
 
 ## Source compatibility