Update Visibility docs on maintainSize

[younghwan]

Description

To make the documentation clearer, I added a statement about maintainState

Related Issues

Fixes #53073

Tests

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[goderbauer]

LGTM

[feinstein]

This doesn't completely fixes the issue I opened, it just adds one of the recommendations I gave.

There's a also the problem with:

  /// Maintaining the size when the widget is not [visible] is not notably more
  /// expensive than just keeping animations running without maintaining the
  /// size, and may in some circumstances be slightly cheaper if the subtree is
  /// simple and the [visible] property is frequently toggled, since it avoids
  /// triggering a layout change when the [visible] property is toggled. If the
  /// [child] subtree is not trivial then it is significantly cheaper to not
  /// even keep the state (see [maintainState]).

Which goes in depth to explain the costs of maintaining the size, animations and state, but we don't actually have an option (and the text makes it look like we have) as if we select to mantainSize we are obligated to also add maintainAnimations and mantainState.

That's the part that I think needs clarification. It looks like something that was written a while ago, but the Framework evolved and added constraints that the docs don't fit anymore.