Sections in let-declarators, for documentation

[AshleyYakeley]

Something like:

let

a = 1;

#| description of important stuff
section "Important Stuff" of
    b = 2;
    c = 3;
end;

d = 4;

in body

• This would only affect documentation, not program behaviour.
• let only, not let rec.
• Namespaces would not create sections unless you do namespace section MyNamespace of, etc.

[AshleyYakeley]

Done.