add explanation of default handling to ICP Config#90

LowriJenkins wants to merge 2 commits intomasterISISComputingGroup/ibex_developers_manual:masterfrom

Ticket_8445_single_source_of_truth_for_macro_defaultsISISComputingGroup/ibex_developers_manual:Ticket_8445_single_source_of_truth_for_macro_defaultsCopy head branch name to clipboard