bottom_sheet: Clarify doc about behavior when useSafeArea is false (#134793)

As discussed at https://github.com/flutter/flutter/pull/122225#issuecomment-1690729628, this is a docs change meant to help people in the absence of a fix for #121752, which is being closed as WONTFIX.

packages/flutter/lib/src/material/bottom_sheet.dart

@@ -792,8 +792,10 @@ class _ModalBottomSheetState<T> extends State<_ModalBottomSheet<T>> {
 /// dragged up and down and dismissed by swiping downwards.
 ///
 /// The [useSafeArea] parameter specifies whether the sheet will avoid system
-/// intrusions on the top, left, and right. If false, no SafeArea is added
-/// and the top padding is consumed using [MediaQuery.removePadding].
+/// intrusions on the top, left, and right. If false, no [SafeArea] is added;
+/// and [MediaQuery.removePadding] is applied to the top,
+/// so that system intrusions at the top will not be avoided by a [SafeArea]
+/// inside the bottom sheet either.
 /// Defaults to false.
 ///
 /// The optional [backgroundColor], [elevation], [shape], [clipBehavior],
@@ -979,8 +981,14 @@ class ModalBottomSheetRoute<T> extends PopupRoute<T> {
   /// If true, a [SafeArea] is inserted to keep the bottom sheet away from
   /// system intrusions at the top, left, and right sides of the screen.
   ///
-  /// If false, the bottom sheet isn't exposed to the top padding of the
-  /// MediaQuery.
+  /// If false, the bottom sheet will extend through any system intrusions
+  /// at the top, left, and right.
+  ///
+  /// If false, then moreover [MediaQuery.removePadding] will be used
+  /// to remove top padding, so that a [SafeArea] widget inside the bottom
+  /// sheet will have no effect at the top edge. If this is undesired, consider
+  /// setting [useSafeArea] to true. Alternatively, wrap the [SafeArea] in a
+  /// [MediaQuery] that restates an ambient [MediaQueryData] from outside [builder].
   ///
   /// In either case, the bottom sheet extends all the way to the bottom of
   /// the screen, including any system intrusions.