docs(proposal): LE-003 clarification

Author: vknabel

proposals/LE-003-string-concatenation.md

@@ -29,9 +29,12 @@ This allows comparisions of concatenated strings.
 
 ## Changes to the Standard Library
 
-Only implementations are affected.
 `strings.concat` will still be available, and will never be deprecated.
 
+Implementations of string related functions should use `..` internally.
+
+Deprecation of `String.append`.
+
 ## Alternatives Considered
 
 We could overload `+`. But due to the dynamic typing it may be confusing in practice.