manpage for zpool does not document all flags for clear
manpage - manual pages
Fragment from the manpage
zpool clear pool [device] Clears device errors in a pool. If no arguments are specified, all device errors within the pool are cleared. If one or more devices is specified, only those errors associated with the specified device or devices are cleared.
Output when running 'zpool clear pool dev1 dev2'
too many arguments usage: clear [-nF] <pool> [device]
(It takes only once device, but that aside ...)
De -n and -F flags are not document.