nixos: boot.zfsImportAll = false; by default

Also add 21.03 release note

nixos/doc/manual/release-notes/rl-2103.xml

@@ -151,6 +151,15 @@
       <literal>vim</literal> switched to Python 3, dropping all Python 2 support.
     </para>
    </listitem>
+   <listitem>
+     <para>
+      <link linkend="opt-boot.zfs.forceImportAll">boot.zfs.forceImportAll</link>
+      previously did nothing, but has been fixed. However its default has been
+      changed to <literal>false</literal> to preserve the existing default
+      behaviour. If you have this explicitly set to <literal>true</literal>,
+      please note that your non-root pools will now be forcibly imported.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 

nixos/modules/tasks/filesystems/zfs.nix

@@ -175,14 +175,10 @@ in
 
       forceImportAll = mkOption {
         type = types.bool;
-        default = true;
+        default = false;
         description = ''
           Forcibly import all ZFS pool(s).
 
-          This is enabled by default for backwards compatibility purposes, but it is highly
-          recommended to disable this option, as it bypasses some of the safeguards ZFS uses
-          to protect your ZFS pools.
-
           If you set this option to <literal>false</literal> and NixOS subsequently fails to
           import your non-root ZFS pool(s), you should manually import each pool with
           "zpool import -f &lt;pool-name&gt;", and then reboot. You should only need to do