orgstruct++-mode: Make more visible in docs, parse item body

orgstruct++-mode: Make more visible in docs, parse item body

orgstruct++-mode is an enhanced version of orgstruct mode that
also imports all indentation and paragraph settings into the major
mode.  Furthermore, it now allows to use M-RET and M-S-RET in items
after the first line.  The latter change was a request by Austin
Frank.