branches/zip: FIL_PAGE_FILE_FLUSH_LSN: Note that the field is only valid

for the first page of each ibdata* file, not *.ibd files.
Suggested by Heikki, in connection with the LSN warning noted in Issue #341.

include/fil0fil.h

@@ -110,9 +110,10 @@ extern fil_addr_t	fil_addr_null;
 					contents of this field is valid
 					for all uncompressed pages. */
 #define FIL_PAGE_FILE_FLUSH_LSN	26	/*!< this is only defined for the
-					first page in a data file: the file
-					has been flushed to disk at least up
-					to this lsn */
+					first page in a system tablespace
+					data file (ibdata*, not *.ibd):
+					the file has been flushed to disk
+					at least up to this lsn */
 #define FIL_PAGE_ARCH_LOG_NO_OR_SPACE_ID  34 /*!< starting from 4.1.x this
 					contains the space id of the page */
 #define FIL_PAGE_DATA		38	/*!< start of the data on the page */