Document segments and debug env var in codegen.
Document segments and debug env var in codegen.

shssoichiro/Rocket,gsquire/Rocket,antonpirker/Rocket,shssoichiro/Rocket,impowski/Rocket,antonpirker/Rocket,gsquire/Rocket,impowski/Rocket,gsquire/Rocket,impowski/Rocket,antonpirker/Rocket,shssoichiro/Rocket