[AssumeBundle] Add documentation for the operand bundles of an llvm.assume

Summary:
Operand bundles on an llvm.assume allows representing
assumptions that an attribute holds for a certain value at a certain position.
Operand bundles enable assumptions that are either hard or impossible to
represent as a boolean argument of an llvm.assume.

Reviewers: jdoerfert, fhahn, nlopes, reames, regehr, efriedma

Reviewed By: jdoerfert

Subscribers: lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74209