[documentation] remove repeated "For example," in RenderSliverEdgeInsetsPadding documentation (#135297)

There was a repeated "For example," in the documentation of `RenderSliverEdgeInsetsPadding`. This PR fix this.

## Description

This removes all of the comments that are of the form "so-and-so (must not be null|can ?not be null|must be non-null)" from the cases where those values are defines as non-nullable values.

This PR removes them from the painting and rendering libraries.

This was done by hand, since it really didn't lend itself to scripting, so it needs to be more than just spot-checked, I think. I was careful to leave any comment that referred to parameters that were nullable, but I may have missed some.

In addition to being no longer relevant after null safety has been made the default, these comments were largely fragile, in that it was easy for them to get out of date, and not be accurate anymore anyhow.

This did create a number of constructor comments which basically say "Creates a [Foo].", but I don't really know how to avoid that in a large scale change, since there's not much you can really say in a lot of cases.  I think we might consider some leniency for constructors to the "Comment must be meaningful" style guidance (which we de facto have already, since there are a bunch of these).

## Related PRs
- https://github.com/flutter/flutter/pull/134984
- https://github.com/flutter/flutter/pull/134991
- https://github.com/flutter/flutter/pull/134992
- https://github.com/flutter/flutter/pull/134994

## Tests
 - Documentation only change.

Remove 1745 decorative breaks

* Use `curly_braces_in_flow_control_structures` for `rendering`

* fix comments

* fix dangling

* apply upcoming lint cast_nullable_to_non_nullable

* make RenderObjectElement.renderObject non-nullable

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This reverts commit 2132a0c7.

This reverts commit 69080e07.

* implicit-casts:false in flutter/lib/src/rendering

* address review comments

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Fix SliverPadding dropping the chain on scrollOffsetCorrection propagation

* review doc

* Fix the confusing-zero case with NestedScrollView.

* Update mock_canvas.dart

* Update tabs_demo.dart

* more tweaks

This patch includes:

 * SliverPadding
 * ScrollView
 * PageView

Fixes #11764
Fixes #11836

Fix sliver padding assertion failure that sometimes happens during startup because the app is temporarily built with zero size

Also, fix some logic in SliverPadding.

* add @required when there's an assert not null

* address review comments

Also, add a few other random dartdocs that I encountered along the way.

Previously the "left" padding was applied on the right when hit testing
a reversed vertical list view.

...for the case where there's no right or bottom padding.