Add description of default sorting behavior to zfs_list.8

The sorting logic is all in cmd/zfs/zfs_iter.c.  I borrowed
where I could from the comments in the source code, but please
note that the comment to zfs_sort() is a little imprecise, or at
least incomplete, because it doesn't give any indication of the
chronological sort that will be used by default for snapshots in
zfs_compare().

While adding this description, I took the liberty to copy-edit
the rest of the file lightly.

In those edits, I've removed "If specified, you can list
property information by the absolute pathname or the relative
pathname" because, in context, it seems more confusing than
helpful.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Shawn Bayern <sbayern@law.fsu.edu>
Closes #15713
Closes #15869

man/man8/zfs-list.8

@@ -50,27 +50,25 @@
 .Oo Ar filesystem Ns | Ns Ar volume Ns | Ns Ar snapshot Oc Ns …
 .
 .Sh DESCRIPTION
-If specified, you can list property information by the absolute pathname or the
+By default, all file systems and volumes are displayed, with the following
-relative pathname.
+fields:
-By default, all file systems and volumes are displayed.
+.Sy name , Sy used , Sy available , Sy referenced , Sy mountpoint .
 Snapshots are displayed if the
 .Sy listsnapshots
 pool property is
 .Sy on
 .Po the default is
 .Sy off
-.Pc ,
+.Pc
 or if the
 .Fl t Sy snapshot
 or
 .Fl t Sy all
 options are specified.
-The following fields are displayed:
-.Sy name , Sy used , Sy available , Sy referenced , Sy mountpoint .
 .Bl -tag -width "-H"
 .It Fl H
 Used for scripting mode.
-Do not print headers and separate fields by a single tab instead of arbitrary
+Do not print headers, and separate fields by a single tab instead of arbitrary
 white space.
 .It Fl j , -json Op Ar --json-int
 Print the output in JSON format.
@@ -87,7 +85,7 @@ of
 will display only the dataset and its direct children.
 .It Fl o Ar property
 A comma-separated list of properties to display.
-The property must be:
+Each property must be:
 .Bl -bullet -compact
 .It
 One of the properties described in the
@@ -125,30 +123,41 @@ section of
 or the value
 .Sy name
 to sort by the dataset name.
-Multiple properties can be specified at one time using multiple
+Multiple properties can be specified to operate together using multiple
 .Fl s
-property options.
+or
+.Fl S
+options.
 Multiple
 .Fl s
-options are evaluated from left to right in decreasing order of importance.
+and
-The following is a list of sorting criteria:
+.Fl S
+options are evaluated from left to right to supply sort keys in
+decreasing order of priority.
+Property types operate as follows:
 .Bl -bullet -compact
 .It
 Numeric types sort in numeric order.
 .It
 String types sort in alphabetical order.
 .It
-Types inappropriate for a row sort that row to the literal bottom, regardless of
+Types inappropriate for a row sort that row to the literal bottom,
-the specified ordering.
+regardless of the specified ordering.
 .El
 .Pp
-If no sorting options are specified the existing behavior of
+If no sort columns are specified, or if two lines of output would sort
-.Nm zfs Cm list
+equally across all specified columns, then datasets and bookmarks are
-is preserved.
+sorted by name, whereas snapshots are sorted first by the name of their
+dataset and then by the time of their creation.
+When no sort columns are specified but snapshots are listed, this
+default behavior causes snapshots to be grouped under their datasets in
+chronological order by creation time.
 .It Fl S Ar property
 Same as
 .Fl s ,
-but sorts by property in descending order.
+but sorts by
+.Ar property
+in descending order.
 .It Fl t Ar type
 A comma-separated list of types to display, where
 .Ar type