[GlobalISel] Update the documentation and comment for G_[UN]MERGE_VALUES

In r296921, we added the G_[UN]MERGE_VALUES node, but did not update the
documentation. Fixing that.

NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314168 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed