add comment clarifying busybox's use of non-standard tar header

archival/libunarchive/get_header_tar.c

@@ -61,6 +61,9 @@ char get_header_tar(archive_handle_t *archive_handle)
 		char linkname[100]; /* 157-256 */
 		/* POSIX:   "ustar" NUL "00" */
 		/* GNU tar: "ustar  " NUL */
+		/* Normally it's defined as magic[6] followed by
+		 * version[2], but we put them together to save code.
+		 */
 		char magic[8];      /* 257-264 */
 		char uname[32];     /* 265-296 */
 		char gname[32];     /* 297-328 */