Fix the BigDecimal documentation to not be a copy & paste of the BigInteger documentation.

Also do some other documentation clean-up. In particular, it's confusing
to use ^ to mean exponentiation when the language uses it to mean
exclusive-or.

(cherry-pick of 995e3c8d0f5dc43bc8ba5f305d1525210caaf796.)

Bug: https://code.google.com/p/android/issues/detail?id=54103
Change-Id: I704bb4d29ea27664adcba6579eabbccd3782b6b1
2 files changed