Update docs on Rune.DecodeFromUtf8

This was updated in dotnet/runtime#328

Author: Rob-Hague

xml/System.Text/Rune.xml

@@ -327,7 +327,7 @@ The general convention is to call this method in a loop, slicing the `source` bu
         <returns>
           <see cref="F:System.Buffers.OperationStatus.Done" /> if the source buffer begins with a valid UTF-8 encoded scalar value. <paramref name="result" /> then contains the decoded <see cref="T:System.Text.Rune" />, and <paramref name="bytesConsumed" /> contains the number of <see cref="T:System.Byte" /> values used in the input buffer to encode the <see cref="T:System.Text.Rune" />.
 
-<see cref="F:System.Buffers.OperationStatus.NeedMoreData" /> if the source buffer is empty or contains only a standalone UTF-8 high surrogate character. <paramref name="result" /> then contains <see cref="P:System.Text.Rune.ReplacementChar" />, and <paramref name="bytesConsumed" /> contains the length of the input buffer.
+<see cref="F:System.Buffers.OperationStatus.NeedMoreData" /> if the source buffer is empty or contains only a partial UTF-8 subsequence. <paramref name="result" /> then contains <see cref="P:System.Text.Rune.ReplacementChar" />, and <paramref name="bytesConsumed" /> contains the length of the input buffer.
 
 <see cref="F:System.Buffers.OperationStatus.InvalidData" /> if the source buffer begins with an ill-formed UTF-8 encoded scalar value. <paramref name="result" /> then contains <see cref="P:System.Text.Rune.ReplacementChar" />, and <paramref name="bytesConsumed" /> contains the number of <see cref="T:System.Byte" /> values used in the input buffer to encode the ill-formed sequence.
           .</returns>