Improve documentation for utf8 decoder (and BOMs).

sdk/lib/convert/utf.dart

Index: sdk/lib/convert/utf.dart
diff --git a/sdk/lib/convert/utf.dart b/sdk/lib/convert/utf.dart
index 6b9feceaa7b9e703ca77fc6cf3cee6a967597ee1..86a34af5abca855774594bf87a4fb577146cd795 100644
--- a/sdk/lib/convert/utf.dart
+++ b/sdk/lib/convert/utf.dart
@@ -330,8 +330,8 @@ class Utf8Decoder extends Converter<List<int>, String>
    * Uses the code units from [start] to, but no including, [end].
    * If [end] is omitted, it defaults to `codeUnits.length`.
    *
-   * If the [codeUnits] start with a leading [UNICODE_BOM_CHARACTER_RUNE] this
-   * character is discarded.
+   * If the [codeUnits] start with the encoding of a
+   * [UNICODE_BOM_CHARACTER_RUNE], that character is discarded.
    */
   String convert(List<int> codeUnits, [int start = 0, int end]) {
     // Allow the implementation to intercept and specialize based on the type