zfs.4: document five missing module parameters

Add entries for module parameters that are exposed via
ZFS_MODULE_PARAM but not covered in zfs.4:

  zfs_active_allocator          (charp,  module/zfs/metaslab.c)
  zfs_compressed_arc_enabled    (int,    module/zfs/arc.c)
  zfs_arc_no_grow_shift         (uint,   module/os/freebsd/zfs/arc_os.c)
  zfs_scan_blkstats             (int,    module/zfs/dsl_scan.c)
  zfs_snapshot_history_enabled  (int,    module/zfs/dsl_dataset.c)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Christos Longros <chris.longros@gmail.com>
Closes #18456

man/man4/zfs.4

@@ -745,6 +745,13 @@ depends on kernel configuration.
 This is the minimum allocation size that will use scatter (page-based) ABDs.
 Smaller allocations will use linear ABDs.
 .
+.It Sy zfs_active_allocator Ns = Ns Sy dynamic Pq charp
+Select the SPA metaslab allocator.
+Valid values are
+.Sy dynamic
+and
+.Sy cursor .
+.
 .It Sy zfs_arc_dnode_limit Ns = Ns Sy 0 Ns B Pq u64
 When the number of bytes consumed by dnodes in the ARC exceeds this number of
 bytes, try to unpin some of it in response to demand for non-metadata.
@@ -937,6 +944,13 @@ If
 equivalent to the greater of the number of online CPUs and
 .Sy 4 .
 .
+.It Sy zfs_arc_no_grow_shift Ns = Ns Sy 5 Pq uint
+If less than
+.Sy arc_c No >> Sy zfs_arc_no_grow_shift
+free memory is available, the ARC is not allowed to grow.
+This parameter is
+.Fx Ns -specific .
+.
 .It Sy zfs_arc_overflow_shift Ns = Ns Sy 8 Pq int
 The ARC size is considered to be overflowing if it exceeds the current
 ARC target size
@@ -1034,6 +1048,11 @@ The timeout is scaled based on a percentage of the last lwb
 latency to avoid significantly impacting the latency of each individual
 transaction record (itx).
 .
+.It Sy zfs_compressed_arc_enabled Ns = Ns Sy 1 Ns | Ns 0 Pq int
+Enables storing ARC buffers in their on-disk compressed form, reducing
+memory pressure.
+When disabled, buffers are decompressed before being cached.
+.
 .It Sy zfs_condense_indirect_commit_entry_delay_ms Ns = Ns Sy 0 Ns ms Pq int
 Vdev indirection layer (used for device removal) sleeps for this many
 milliseconds during mapping generation.
@@ -1690,6 +1709,11 @@ which have the
 .Em no_root_squash
 option set.
 .
+.It Sy zfs_snapshot_history_enabled Ns = Ns Sy 1 Ns | Ns 0 Pq int
+Whether snapshot creation and destruction events are recorded in the pool
+history log, viewable with
+.Nm zpool Cm history .
+.
 .It Sy zfs_snapshot_no_setuid Ns = Ns Sy 0 Ns | Ns 1 Pq int
 Whether to disable
 .Em setuid/setgid
@@ -2181,6 +2205,10 @@ working on a scrub between TXG flushes.
 .It Sy zfs_scrub_error_blocks_per_txg Ns = Ns Sy 4096 Pq uint
 Error blocks to be scrubbed in one txg.
 .
+.It Sy zfs_scan_blkstats Ns = Ns Sy 0 Ns | Ns 1 Pq int
+When enabled, ZFS counts blocks by type and indirection level during a scrub.
+The counts are kept in memory for debugging and are not exposed to userspace.
+.
 .It Sy zfs_scan_checkpoint_intval Ns = Ns Sy 7200 Ns s Po 2 hour Pc Pq uint
 To preserve progress across reboots, the sequential scan algorithm periodically
 needs to stop metadata scanning and issue all the verification I/O to disk.