[Doc] Fix getelementptr description about arguments

Section "Arguments" of `getelementptr` [1] says the first argument is a
type, the second argument is a pointer or a vector of pointers, and is
the base address to start from. Update `getelementptr` FAQ [2]
accordingly, based on discussion with David on the mailing list [3].

[1] http://llvm.org/docs/LangRef.html#getelementptr-instruction
[2] http://llvm.org/docs/GetElementPtr.html
[3] http://lists.llvm.org/pipermail/llvm-dev/2017-June/114294.html

Patch by Wei-Ren Chen!

Differential Revision: https://reviews.llvm.org/D34325

llvm-svn: 305662