Make reinit public as g:MC_ReInitBufferForAllMatchControls

If the autocommands don't trigger for buffers created by, for example,
plug-ins, you can now script around it and use this public function to
set all match controls to the configured default state.
README: overall improvements

Copy applicable documentation from vim-excess-lines here and adapt it.
I consider EL as mostly obsolete.  Improvements will end up here.