READ_PLUS: Remove text about di_reserved

READ_PLUS no longer has a di_reserved field, so this text should be
removed.  I also reordered the sentences in this area to put
hole-specific text into a single paragraph.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Tom Haynes <Thomas.Haynes@primarydata.com>