I guess in theory there is nothing stopping the system from providing yet other files, like say "/proc/123/ctl/description" which provides a human-readable desc

I guess in theory there is nothing stopping the system from providing yet other files, like say "/proc/123/ctl/description" which provides a human-readable description of what "/proc/123/ctl" does, and "/proc/123/ctl/schema" which provides a standardized schema for interacting with the parent, akin to a XSD or JSON Schema.