BUG#27850 "mysqldump --single-transaction does not work if TRUNCATE/DROP/RENAME/ALTER":

this has already been closed by a note in the manual, now we
complete this by a note in "mysqldump --help"

client/mysqldump.c

@@ -404,8 +404,12 @@ static struct my_option my_long_options[] =
    "Creates a consistent snapshot by dumping all tables in a single "
    "transaction. Works ONLY for tables stored in storage engines which "
    "support multiversioning (currently only InnoDB does); the dump is NOT "
-   "guaranteed to be consistent for other storage engines. Option "
-   "automatically turns off --lock-tables.",
+   "guaranteed to be consistent for other storage engines. "
+   "While a --single-transaction dump is in process, to ensure a valid "
+   "dump file (correct table contents and binary log position), no other "
+   "connection should use the following statements: ALTER TABLE, DROP "
+   "TABLE, RENAME TABLE, TRUNCATE TABLE, as consistent snapshot is not "
+   "isolated from them. Option automatically turns off --lock-tables.",
    (gptr*) &opt_single_transaction, (gptr*) &opt_single_transaction, 0,
    GET_BOOL, NO_ARG,  0, 0, 0, 0, 0, 0},
   {"skip-opt", OPT_SKIP_OPTIMIZATION,