Update man page about zfs_arc_shrinker_limit ignore

Signed-off-by: Gionatan Danti <[email protected]>

man/man4/zfs.4

@@ -820,7 +820,9 @@ only operates during memory pressure/reclaim.
 This is a limit on how many pages the ARC shrinker makes available for
 eviction in response to one page allocation attempt.
 Note that in practice, the kernel's shrinker can ask us to evict
-up to about four times this for one allocation attempt.
+up to about four times this for one allocation attempt. To limit
+OOM risk, this parameters is ignored if the kernel is in direct
+reclaim mode.
 .Pp
 The default limit of
 .Sy 10000 Pq in practice, Em 160 MiB No per allocation attempt with 4 KiB pages