Shifted early documentation to @{20-main/help}.

Awkward early documentation has now been shifted out of @{00-core/00-declare},
rendering such parcels somewhat more concise and readily maintainable.
Likewise, define_function() and redefine_function() have been shifted from such
tree to @{00-core/40-define}, a new parcel providing runnable definition
functionality. redefine_function() has been improved for both efficiency and
maintainability, eliminating eggregious pipes and related commentary.