I've never heard of any formal system like that. Shell scripting isn't generally as complex as using a full programming language, as it's mostly just a way to string together various other programs in a useful way. Since most shell scripts are created as quick-and-dirty ways to solve an immediate need, there's generally not much call for a complex documentation system. Script "documentation" is usually just whatever output you want to echo to standard out, or add as comments inside the script itself.
OTOH, a quick search of apt turned up robodoc
, which says it can be used with shell scripts. Perhaps that's the kind of thing you're looking for.