consolidate xz format comment. no code changes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/unarchive.h

@@ -9,6 +9,8 @@ enum {
 	COMPRESS_MAGIC = 0x1f9d,
 	GZIP_MAGIC  = 0x1f8b,
 	BZIP2_MAGIC = 'B' * 256 + 'Z',
+	/* .xz signature: 0xfd, '7', 'z', 'X', 'Z', 0x00 */
+	/* More info at: http://tukaani.org/xz/xz-file-format.txt */
 	XZ_MAGIC1   = 0xfd * 256 + '7',
 	XZ_MAGIC2   = (('z' * 256 + 'X') * 256 + 'Z') * 256 + 0,
 	/* Different form: 32 bits, then 16 bits: */