Clarify size and offset of tags. Reported by: Seth Goldberg

doc/multiboot.texi

@@ -371,7 +371,7 @@ The layout of the Multiboot header must be as follows:
 @item 4      @tab u32 @tab architecture   @tab required
 @item 8      @tab u32 @tab header_length  @tab required
 @item 12     @tab u32 @tab checksum       @tab required
-@item 16     @tab u32 @tab tags           @tab required
+@item 16-XX  @tab     @tab tags           @tab required
 @end multitable
 
 The fields @samp{magic}, @samp{architecture}, @samp{header_length} and @samp{checksum} are defined in @ref{Header magic fields}, @samp{tags}