WL#8845: Clarify the message about redo log format incompatibility

recv_find_max_checkpoint(): Refer to MariaDB 10.2.2 instead of
MySQL 5.7.9. Do not hint that a binary downgrade might be possible,
because there are many changes in InnoDB 5.7 that could make
downgrade impossible: a column appended to SYS_INDEXES, added
SYS_* tables, undo log format changes, and so on.

storage/innobase/log/log0recv.cc

@@ -818,10 +818,8 @@ recv_find_max_checkpoint(
 
 		fprintf(stderr,
 			"InnoDB: No valid checkpoint found.\n"
-			"InnoDB: If you are attempting downgrade"
-			" from MySQL 5.7.9 or later,\n"
-			"InnoDB: please refer to " REFMAN
-			"upgrading-downgrading.html\n"
+			"InnoDB: A downgrade from MariaDB 10.2.2"
+			" or later is not supported.\n"
 			"InnoDB: If this error appears when you are"
 			" creating an InnoDB database,\n"
 			"InnoDB: the problem may be that during"

storage/xtradb/log/log0recv.cc

@@ -845,10 +845,8 @@ recv_find_max_checkpoint(
 
 		fprintf(stderr,
 			"InnoDB: No valid checkpoint found.\n"
-			"InnoDB: If you are attempting downgrade"
-			" from MySQL 5.7.9 or later,\n"
-			"InnoDB: please refer to " REFMAN
-			"upgrading-downgrading.html\n"
+			"InnoDB: A downgrade from MariaDB 10.2.2"
+			" or later is not supported.\n"
 			"InnoDB: If this error appears when you are"
 			" creating an InnoDB database,\n"
 			"InnoDB: the problem may be that during"