String.lastIndexOf documentation should clarify what position is returned

[jamesderlin]

https://api.dart.dev/stable/2.8.2/dart-core/String/lastIndexOf.html states:

Returns the position of the last match pattern in this string, searching backward starting at start, inclusive:

Is 'foobar'.lastIndexOf('foo') supposed to return 0 or 2?

I think that this would be clearer as something like "Returns the starting position".

Additionally, the examples unfortunately happen to use search patterns that match only a single character. The lone example that uses a longer search pattern is for a case where the string isn't found. Changing the examples to use longer search patterns also would clarify the expected behavior.