BigDecimal doc fixes

This introduces minor fixes/changes to `BigDecimal` documentation:

* `BigDecimal#==` returns `false`, not `nil`.
* `10_000.times do` seems more idiomatic (and more readable in the case of this example) than `for i in (1..10000)`.
* There’s a difference in specificity between what mathematical ‘infinity’ and `BigDecimal`’s +Infinity and -Infinity are; this tries to adjust the spelling in the right places.