Documentation: No overflow-checking in accumulators, no precision guarantees

[Tim Odenthal]

As discussed here​http://old.nabble.com/-accumulators--Is-there-a-reset-function-yet--td28635232.html, I suggest changing the documentation to warn users of possible overflow/loss of precision issues:

Maybe in the section "The Accumulators Framework" one could add the sentence

To ensure maximum performance, the predefined accumulators in the library do not check for possible overflows or loss of precision by adding numbers of very different exponents.