Document running nixos-rebuild switch to clear /boot space

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