restore the old rendering of structure field definitions

Restore the rendering of the on-disk struct fields to resemble that of
the old SGI book, with bold field names on one line and the
description indented below.  This is a lot more readable than letting
the whole field and description run together as one long line.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>