SI-6946, SI-6924 Greatly improves IsTraversableLike docs

Given:
1. SI-6924 Error in IsTraversableLike scaladoc example (example code did not compile)
2. SI-6946 Documentation for IsTraversableLike is confusing (my dissatisfaction with the docs for IsTraversableLike)

I rewrote the docs completely and provided an alternate example. While the fix provided in #1850 did indeed make the example code compile, the example code was still far too complicated to clearly illustrate an already confusing concept.

Funny observation: the amount of docs now greatly exceeds the amount of code.

Unfortunately, this renders PR #1850 now unnecessary.

Review by @jsuereth and @phaller