Rewrite the help pages for reading results to tune's length, structure and voice

[jmgirard]

The 24 help pages for reading a nested run now follow the shape of tune's help pages. They cover the collect, compute and augment methods, agreement(), autoplot(), selection_rule(), the extractors, and the print, summary and predict methods.

• Together the pages have 4,385 prose words, down from 9,116, and each Description is 15 to 37 words.
• The condition class names and the lists of errors left these pages.
• Each print and summary page says that the nested estimate describes the tuning procedure, not a fitted model. ?collect_metrics.nested_results says what to report and when.
• ?collect_metrics.nested_results states the standard error in three sentences and points to vignette("estimate").
• One sentence in vignettes/results.Rmd pointed to a warning class that left ?collect_selections, so it is deleted.

devtools::test() passes, and devtools::check() gives 0 errors, 0 warnings and 0 notes.

Refs #91