Add emphasis on the difference between imageIndex and currentFrame

I've witnessed people in the past having a hard time understanding the difference between the swap chain image index and the index you would use for your frames in flight. This tutorial does properly separate the two, but I feel like it needs a stronger emphasis on the difference between them, to make it less easy to screw up.

I think [this part](https://github.com/KhronosGroup/Vulkan-Tutorial/blob/main/en/03_Drawing_a_triangle/03_Drawing/03_Frames_in_flight.adoc) is a great place to include such information, since this is where the concept of FiF is introduced.

Also this is something I'd be happy to make a PR for

Start by reading en/03_Drawing_a_triangle/03_Drawing/03_Frames_in_flight.adoc, especially where frames in flight are introduced. Clarify the distinction between the swap chain image index and the frame-in-flight index in that section; the work is done when the two indices and their separate uses are unambiguous to readers.

Written by the indexing model from the issue text.