Update LangRef for getelementptr explicit type changes

Update LangRef for getelementptr explicit type changes

Here's a rough/first draft - it at least hits the actual textual IR
examples and some of the phrasing. It's probably worth a full pass over,
but I'm not sure how much these docs should reflect the strange
intermediate state we're in anyway.

Totally open to lots of review/feedback/suggestions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231294 91177308-0d34-0410-b5e6-96231b3b80d8