doc: modify param decription for end(),write() in StringDecoder

Author: pluris

doc/api/string_decoder.md

@@ -63,7 +63,7 @@ Creates a new `StringDecoder` instance.
 added: v0.9.3
 -->
 
-* `buffer` {Buffer|TypedArray|DataView} A `Buffer`, or `TypedArray`, or
+* `buffer` {string|Buffer|TypedArray|DataView} A `Buffer`, or `TypedArray`, or
   `DataView` containing the bytes to decode.
 * Returns: {string}
 
@@ -86,7 +86,7 @@ changes:
                  character instead of one for each individual byte.
 -->
 
-* `buffer` {Buffer|TypedArray|DataView} A `Buffer`, or `TypedArray`, or
+* `buffer` {string|Buffer|TypedArray|DataView} A `Buffer`, or `TypedArray`, or
   `DataView` containing the bytes to decode.
 * Returns: {string}