doc: update that ascii write doesn't convert null

Since WriteBuffer has been replaced with WriteOneByte, writing ascii
will no longer automatically convert 0x0 to 0x20. So removed mention of
this special case from docs.

doc/api/buffer.markdown

@@ -20,10 +20,6 @@ encoding method.  Here are the different string encodings.
 * `'ascii'` - for 7 bit ASCII data only.  This encoding method is very fast, and
   will strip the high bit if set.
 
-  Note that when converting from string to buffer, this encoding converts a null
-  character (`'\0'` or `'\u0000'`) into `0x20` (character code of a space). If
-  you want to convert a null character into `0x00`, you should use `'utf8'`.
-
 * `'utf8'` - Multibyte encoded Unicode characters. Many web pages and other
   document formats use UTF-8.