Document how users can set debug flags in their code if they wish to persist debugging options

One user on the user survey requested that we set the debugging options like track widget builds by default. We cannot do this due to the performance overhead of these debugging options. What we can do though, is improve documentation to tell users how they can set these flags from their code if they wish to have these settings persist across app runs.

Start by locating the existing DevTools documentation for debugging options and the references to flags such as track widget builds. Document how users can set these flags from application code so the settings persist across app runs, and explain why they are not enabled by default. Done means the relevant options and persistence steps are clearly documented.

Escrito por el modelo de indexación a partir del texto del issue.