Improve "repmgr primary unregister" documentation and --help output

Per observations in GitHub #373

doc/repmgr-primary-unregister.sgml

@@ -21,6 +21,10 @@
 
   <refsect1>
     <title>Execution</title>
+    <para>
+      <command>repmgr primary unregister</command> should be run on the current primary,
+      with the ID of the node to unregister passed as <option>--node-id</option>.
+    </para>
     <para>
       Execute with the <literal>--dry-run</literal> option to check what would happen without
       actually unregistering the node.
@@ -32,6 +36,34 @@
     </para>
   </refsect1>
 
+  <refsect1>
+
+    <title>Options</title>
+
+    <variablelist>
+
+      <varlistentry>
+        <term><option>--dry-run</option></term>
+        <listitem>
+          <para>
+            Check prerequisites but don't actually unregister the primary.
+          </para>
+        </listitem>
+      </varlistentry>
+
+     <varlistentry>
+        <term><option>--node-id</option></term>
+        <listitem>
+          <para>
+            ID of the inactive primary to be unregistered.
+          </para>
+        </listitem>
+      </varlistentry>
+
+    </variablelist>
+
+  </refsect1>
+
   <refsect1>
     <title>Event notifications</title>
     <para>

repmgr-action-primary.c

@@ -548,7 +548,8 @@ do_primary_help(void)
 	printf(_("  \"primary unregister\" unregisters an inactive primary node.\n"));
 	puts("");
 	printf(_("  --dry-run                           check what would happen, but don't actually unregister the primary\n"));
-	printf(_("  -F, --force                         force removal of the record\n"));
+	printf(_("  --node-id                           ID of the inactive primary node to unregister.\n"));
+	printf(_("  -F, --force                         force removal of an active record\n"));
 
 	puts("");