Update documentation for Kernel#BigDecimal() digits argument [ci skip]

[jeremyevans]

I expected this to affect the significant digits if providing the first argument as a String, but it does not. I'm positive this argument is ignored if the first argument is an Integer, but I'm not positive that it is ignored if the first argument is a String, as the value is passed to VpAlloc. However, if it has an effect, I'm not sure what it is.