Add docs about Buffer._charsWritten

Fixes GH-907.

doc/api/buffers.markdown

@@ -55,6 +55,10 @@ Example: write a utf8 string into a buffer, then print it
     len = buf.write('\u00bd + \u00bc = \u00be', 0);
     console.log(len + " bytes: " + buf.toString('utf8', 0, len));
 
+The number of characters written (which may be different than the number of
+bytes written) is set in `Buffer._charsWritten` and will be overwritten the
+next time `buf.write()` is called.
+
 
 ### buffer.toString(encoding, start=0, end=buffer.length)