How about this comment:

    // N.B. "GEP" -- GetElementPointer -- is a particular LLVM-ism that is
    // the means for retrieving elements from some kind of aggregate: the
    // i-th field in a struct, the i-th element of an array.  They can be
    // chained together, to get at items in a recursive hierarchy.

    /// Generate a GEP (get element pointer) where the element index is an
    /// llvm::Value, which can be generated from either a constant or a
    /// runtime-computed integer element index.
    llvm::Value *GEP (llvm::Value *ptr, llvm::Value *elem);

    /// Generate a GEP (get element pointer) with two integer element
    /// offsets.  This is just a special (and common) case of GEP where
    /// we have a 2-level hierarchy and we have fixed element indices
    /// that are known at compile time.
    llvm::Value *GEP (llvm::Value *ptr, int elem1, int elem2);