nixos docs: footnotes: give IDs

Changed files
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nixos
doc
manual
configuration
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nixos/doc/manual/configuration/modularity.xml
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argument is for: it contains the complete, merged system configuration. That
is, <varname>config</varname> is the result of combining the configurations
returned by every module
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<footnote>
+
<footnote xml:id="footnote-nix-is-lazy">
<para>
If you’re wondering how it’s possible that the (indirect)
<emphasis>result</emphasis> of a function is passed as an