Add description of `BigDecimal.new` exceptions

`BigDecimal.new` raises some Exceptions.
But it is undocumented.

this PR is very similar to #680.
But #680 seems still not enough. 

Especially, ArgumentError by larger digits value than Float::DIG + 1 causes some trouble.

Will "this exception is raised." go on the same line 2472 without wrap? (We wrap at 80 columns)