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


client/mysqldump.c:
  mention, in "mysqldump --help", of limitations reported as BUG#27850
  "mysqldump --single-transaction does not work if TRUNCATE/DROP/RENAME/ALTER"
  The new text is inspired from what the "mysqldump" section of the
  manual now says about --single-transaction:
  http://dev.mysql.com/doc/refman/5.1/en/mysqldump.html