Update previous tips to reflect guaranteed copy elision in C++17

Many early tips talk about RVO, copy elision etc., but make no mention of C++17. As a result, they often say things like, "Strictly speaking the compiler is not required to perform copy elision".

Now that [C++17 guarantees copy elision in certain situations](https://en.cppreference.com/w/cpp/language/copy_elision), a note can be added to the tips similar to the one given for number 11. It can tell readers to refer to number 166.

These are the tips which I found mention copy elision and were made before C++17:
1. Number [11](https://abseil.io/tips/11)
2. Number [24](https://abseil.io/tips/24)
3. Number [77](https://abseil.io/tips/77)
4. Number [101](https://abseil.io/tips/101)
5. Number [117](https://abseil.io/tips/117)
6. Number [120](https://abseil.io/tips/120)