Add documentation for most of the errors and warnings in analysis options files

We don't have a way to mechanically verify the exampes and fixed code,
so please look to see whether any of it looks wrong. And, of course, let
me know if any of the text appears to be incorrect or could be better.

Change-Id: I9b059524ffcedc1e3b8a7602b7124f419095a702
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/455481
Reviewed-by: Paul Berry <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson