[GlobalISel] Update the documentation and comments for G_EXTRACT

In r297100, G_EXTRACT changed from a multiple results instruction to a
single result one. Update the documentation accordingly.

NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314166 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed