Document of pow() can be more helpful

[ordovicia]

Document of pow() for signed integers and for unsingned integers take 2^4 (^ is not xor but power) as examples.
Because 2^4 = 4^2, these examples are not very helpful for those unfamiliar with math words in English and thus rely on example codes.
We should take another example, e.g. 2^5 (which is not equal to 5^2).