Change Math.pow() test / docs to match actual

The Math.pow() implementation does one thing, the docs say another
and the tests match the docs.

There are good reasons for the behavior so this commit adjusts the
affected docs/test accordingly.

Bug: 11669804
Test: vogar harmony-tests/src/test/java/org/apache/harmony/tests/java/lang/MathTest.java
Change-Id: I727dbd82ca06c9bba3d6e8cec113b21aa1f923bc
3 files changed