Confusing documentation on efficient ADD/MUL invocations.

The documentation for ADD, ADD_D, MUL, and MUL_D all have a confusing paragraph about how best to use them, implying both that "x" should be the smallest and largest parameter for best operation.  This was reported previously here:

https://svn.boost.org/trac/boost/ticket/5580

I came across the same issue today, and after some digging I think that the statement that "x" should be the greater parameter is correct (based on BOOST_PP_WHILE decrementing the second parameter, and thus having that as the smaller one will result in less iterations).  I've fixed the four instances of this error in the documentation that I know of.

Where the changed "(c)" symbol in those commits came from, I don't know (I only just noticed them), but they seem to be changed back to the correct thing.