Man page fixes - zpool/zfs optional parameters

The man pages for zpool and zfs (get command)
listed the pool/dataset parameter as required,
but these are optional. Fixed that.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gregor Kopka <gregor@kopka.net>
Closes #7916

man/man8/zfs.8

@@ -117,7 +117,7 @@
 .Oo Fl s Ar source Ns Oo , Ns Ar source Oc Ns ... Oc
 .Oo Fl t Ar type Ns Oo , Ns Ar type Oc Ns ... Oc
 .Cm all | Ar property Ns Oo , Ns Ar property Oc Ns ...
-.Ar filesystem Ns | Ns Ar volume Ns | Ns Ar snapshot Ns | Ns Ar bookmark Ns ...
+.Oo Ar filesystem Ns | Ns Ar volume Ns | Ns Ar snapshot Ns | Ns Ar bookmark Oc Ns ...
 .Nm
 .Cm inherit
 .Op Fl rS
@@ -2941,7 +2941,7 @@ section.
 .Oo Fl s Ar source Ns Oo , Ns Ar source Oc Ns ... Oc
 .Oo Fl t Ar type Ns Oo , Ns Ar type Oc Ns ... Oc
 .Cm all | Ar property Ns Oo , Ns Ar property Oc Ns ...
-.Ar filesystem Ns | Ns Ar volume Ns | Ns Ar snapshot Ns | Ns Ar bookmark Ns ...
+.Oo Ar filesystem Ns | Ns Ar volume Ns | Ns Ar snapshot Ns | Ns Ar bookmark Oc Ns ...
 .Xc
 Displays properties for the given datasets.
 If no datasets are specified, then the command displays properties for all

man/man8/zpool.8

@@ -83,7 +83,7 @@
 .Op Fl Hp
 .Op Fl o Ar field Ns Oo , Ns Ar field Oc Ns ...
 .Sy all Ns | Ns Ar property Ns Oo , Ns Ar property Oc Ns ...
-.Ar pool Ns ...
+.Oo Ar pool Oc Ns ...
 .Nm
 .Cm history
 .Op Fl il
@@ -1226,7 +1226,7 @@ This may lead to potential data corruption.
 .Op Fl Hp
 .Op Fl o Ar field Ns Oo , Ns Ar field Oc Ns ...
 .Sy all Ns | Ns Ar property Ns Oo , Ns Ar property Oc Ns ...
-.Ar pool Ns ...
+.Oo Ar pool Oc Ns ...
 .Xc
 Retrieves the given list of properties
 .Po