Expand docs for Stream.lastWhere and singleWhere

Closes #30071

Add a template in the `firstWhere` doc for the paragraph about `orElse`
handling. Expand the discussion around error handling in the `lastWhere`
and `singleWhere` docs without relying on reading the `firstWhere` doc
first.

R=lrn@google.com

Change-Id: I302381bb5f4f7daa392ee22bbf667db7d7259ef5
CoreLibraryReviewExempt: Doc changes only.
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/310761
Commit-Queue: Nate Bosch <nbosch@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>

https://dart.googlesource.com/sdk/+/a282120406e22db4651dd5e69a5d9302008eeec5