[Chapel Merge] Add documentation of `bigint.pow` method in BigInt

Branch: refs/heads/main
Revision: 9f92608
Author: lydia-duncan
Log Message:

Merge pull request #18481 from lydia-duncan/documentPow

Add documentation of bigint.pow method in BigInteger module and fix typo
[reviewed by @stonea]

Verified that the built documentation looks good.

While here, fixed a typo I noticed in the bigint.powMod documentation.

Modified Files:
M modules/standard/BigInteger.chpl

Compare: https://github.com/chapel-lang/chapel/compare/8380c64e3e98...9f92608e097f