This function currently returns a non-const reference to the internal vector of operations for a multi-doc transaction. Since the expected usage of this function is to pass along a pointer to the internal operations to the OpObserver methods for committing multi-doc transactions, it would be nice to change the return type of this function from a reference to a pointer.

Making this change will help avoid accidental copies like the scenarios described in SERVER-69478.

This is a slight deviation from our code style for function input and outputs.