nixos manual: add a note about "nofail" FS option

Close #1858, as I think the points have been well resolved.

Changed files
+6
nixos
doc
manual
configuration
+6
nixos/doc/manual/configuration/file-systems.xml
···
<option>fsType</option> to ensure that the kernel module is
available.</para>
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<note><para>System startup will fail if any of the filesystems fails to mount,
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dropping you to the emergency shell.
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You can make a mount asynchronous and non-critical by adding
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<literal>options = [ "nofail" ];</literal>.
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</para></note>
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<xi:include href="luks-file-systems.xml" />
</chapter>