message: announce upcoming breaking changes in sequence
diff: @@ -22,6 +22,34 @@ There are a few steps you can take to write a good change note and avoid needing
  - [Remove `ConnectionState.Connecting`](#Remove-ConnectionState.Connecting)
  - [`IContainerRuntime.flush` is deprecated](#icontainerruntimeflush-is-deprecated)
  - [MergeTree class is deprecated](#MergeTree-class-is-deprecated)
  +- [Various return types in `@fluidframework/sequence` have been widened to include `undefined`](#various-return-types-in-fluidframeworksequence-have-been-widened-to-include-undefined)
  +
  +### Various return types in `@fluidframework/sequence` have been widened to include `undefined`
  +
  +Strict null checks have been enabled in `@fluidframework/sequence`. As part of this, the return types of several functions have been modified to include `| undefined`. This does not represent a behavioral change.
  +
  +The functions affected are:
  + - `Interval.getAdditionalPropertySets`
  + - `Interval.modify`
  + - `IntervalCollection.getIntervalById`
  + - `IntervalCollection.nextInterval`
  + - `IntervalCollection.previousInterval`
  + - `IntervalCollection.removeIntervalById`
  + - `ISharedString.insertMarker`
  + - `PaddingSegment.fromJSONObject`
  + - `RunSegment.createSplitSegmentAt`
  + - `RunSegment.fromJSONObject`
  + - `SequenceEvent.clientId`
  + - `SharedSegmentSequence.getPropertiesAtPosition`
  + - `SharedSegmentSequence.removeLocalReferencePosition`
  + - `SharedSegmentSequence.resolveRemoteClientPosition`
  + - `SharedString.findTile`
  + - `SharedString.getMarkerFromId`
  + - `SharedString.insertMarker`
  + - `SparseMatrix.getItem`
  + - `SparseMatrix.getPositionProperties`
  + - `SubSequence.createSplitSegmentAt`
  + - `SubSequence.fromJSONObject`
  ### Remove `documentId` field from `MockFluidDataStoreContext`
  This field has been deprecated and will be removed in a future breaking change.
  