Update documentation as parallel mark is now on by default in configure

Update documentation as parallel mark is now on by default in configure
(fix commit c7f2147)

* configure.ac (parallel-mark): Provide information about
--disable-parallel-mark option instead of --enable-parallel-mark.
* doc/README.autoconf (Important options): Likewise.
* doc/README.solaris2: Likewise.
* doc/simple_example.html: Likewise.
* doc/README.DGUX386: Remove configure --enable-parallel-mark option
(as it is on by default now).
* doc/README.autoconf (Important options): Do not provide information
about --enable-full-debug which is no longer supported (since v7.0).
* doc/gcinterface.html: update information about parallel marker mode
in configure.
* doc/README.solaris2: Provide information about --disable-threads
option instead of --enable-threads=posix.
* doc/simple_example.html: Likewise.