Document running nixos-rebuild switch to clear /boot space

nixos/doc/manual/administration/cleaning-store.xml

···
   Since this command needs to read the entire Nix store, it can take quite a
   while to finish.
  </para>
+ <section xml:id="sect-nixos-gc-boot-entries">
+  <title>NixOS Boot Entries</title>
+
+  <para>
+   If your <filename>/boot</filename> partition runs out of space, after
+   clearing old profiles you must rebuild your system with
+   <literal>nixos-rebuild</literal> to update the <filename>/boot</filename>
+   partition and clear space.
+  </para>
+ </section>
 </chapter>