Improve dev docs

[rhornung67]

Several items to improve in the Dev Guide:

• Describe recent changes to quiet C++20 compilation errors related use of volatile loop counter variable in reps loops
• Improve description of recent changes to refactor how loop variants and tunings are specified. The code is cleaner, but it is more opaque. In particular, describe the different implementation patterns that appear in different scenarios.

[MrBurmark]

I built #596 with gcc 11.2.1 and found more places where we get warnings if we don't cast the repetition counter increment expression to void.