[Chapel Merge] Add documentation to BigInteger's round enum and d

Branch: refs/heads/main
Revision: 70b424b
Author: lydia-duncan
Log Message:

Merge pull request #18008 from lydia-duncan/documentDivs

Add documentation to BigInteger's round enum and div* functions
[reviewed by @vasslitvinov]

These are a less C-specific version of the documentation from
https://gmplib.org/manual/Integer-Division

Resolves Cray/chapel-private#2130

Double checked that hello.chpl and
test/library/standard/GMP/ldelaney/gmp_Bigint_division.chpl built and ran
appropriately. Built docs also look good.

Modified Files:
M modules/standard/BigInteger.chpl

Compare: https://github.com/chapel-lang/chapel/compare/53b520b49227...70b424bdda98