* don't reference sections that don't exist (typically happens for
registers that don't contain any fields and are interpreted
numerically.
* allow templates to use override_backref for surpressing the list of
registers that use it
* print doc for fields even when it uses values or only exports a mask
* allow fields to define their own mask; those fields have to define
their values explicitly too, and don't have to provide a shift
* don't print a "No values defined for the field" lines, as there's
always a mask in that case by now.
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