Add introductory prose for Data.Traversable
[skip ci] This is a proposed alternative to !3443 (closed) which instead of inline examples for the individual methods adds introductory prose and examples to the bottom of the document, below the function synopses.
In lieu of a screenshot, given the multiple pages of changes, a link to a pre-formatted document makes more sense IMHO.
Tracking: #17929;
/label documentation