Clarify zpool_events_next() comment

Due to the very poorly chosen argument name 'cleanup_fd' it was
completely unclear that this file descriptor is used to track the
current cursor location.  When the file descriptor is created by
opening ZFS_DEV a private cursor is created in the kernel for the
returned file descriptor.  Subsequent calls to zpool_events_next()
and zpool_events_seek() then require the file descriptor as an
argument to reposition the cursor.  When the file descriptor is
closed the kernel state tracking the cursor is destroyed.

This patch contains no functional change, it just changes a
few variable names and clarifies the documentation.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Issue #2

module/zfs/zfs_ioctl.c

@@ -4860,11 +4860,11 @@ zfs_ioc_release(const char *pool, nvlist_t *holds, nvlist_t *errlist)
 /*
  * inputs:
  * zc_guid		flags (ZEVENT_NONBLOCK)
+ * zc_cleanup_fd	zevent file descriptor
  *
  * outputs:
  * zc_nvlist_dst	next nvlist event
  * zc_cookie		dropped events since last get
- * zc_cleanup_fd	cleanup-on-exit file descriptor
  */
 static int
 zfs_ioc_events_next(zfs_cmd_t *zc)