Doc improvements

Fixes #297.

doc/api/buffers.markdown

@@ -16,6 +16,9 @@ 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 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'` - Multi byte encoded Unicode characters.  Many web pages and other document formats use UTF-8.