[boost] Documentation improvement for boost::bind

In the first line of the next to last paragraph of the boost::bind deocumentation under the heading of "Using nested binds for function composition", it is stated: "Sometimes it is necessary not to evaluate the first argument, but not to evaluate some of the other arguments, even when they are nested bind subexpressions. " I believe this is confusing and that a much clearer explanation of what is meant would be: "Although the first argument is, by default, not evaluated, all other arguments are. Sometimes it is necessary not to evaluate arguments subsequent to the first, even when they are nested bind subexpressions."