Use doc formatter when running individual files.
Use groups vs example strategy for groups vs examples.

We were wrongly always using the example strategy
for both groups and examples.
Improve formatting.
Change #[] to #fetch.

For the case of an unrecognized order symbol,
I want to have it take a fallback block
(like the semantics of Hash#fetch), but #[]
can't accept a block so we have to change the API.
Print a warning when given an unrecognized :order option.

Otherwise it may confuse users when they misspell the ordering.
YARD comments for the ordering code.