chore: 8.4.1 hotfix release notes (#18490)

# Overview

Notes for the two bug fixes going into 8.4.1.

## Test Plan and Hands on Testing

n/a

## Changelog

Bullets in `app-shell` release notes and placeholder in `api` release
notes.

## Review requests

Accurate? Especially that there are no robot software changes?

## Risk assessment

none, notes only

Author: ecormany

api/release-notes.md

@@ -8,6 +8,12 @@ By installing and using Opentrons software, you agree to the Opentrons End-User
 
 ---
 
+## Opentrons Robot Software Changes in 8.4.1
+
+There are no changes to the robot software in v8.4.1, but it is required for running protocols in v8.4.1 of the Opentrons App.
+
+---
+
 ## Opentrons Robot Software Changes in 8.4.0
 
 Welcome to the v8.4.0 release of the Opentrons robot software! This release includes updated liquid handling commands for pipetting relative to a liquid meniscus, and other new features, improvements, and bug fixes.

app-shell/build/release-notes.md

@@ -8,6 +8,15 @@ By installing and using Opentrons software, you agree to the Opentrons End-User
 
 ---
 
+## Opentrons App Changes in 8.4.1
+
+The 8.4.1 hotfix release fixes two issues:
+
+- Placing a Magnetic Block in slot C2 no longer prevents Labware Position Check from running.
+- Existing labware offsets are no longer doubled during Labware Position Check.
+
+---
+
 ## Opentrons App Changes in 8.4.0
 
 Welcome to the v8.4.0 release of the Opentrons App! This release includes updates to labware offsets on the Flex, as well as other new features, improvements, and bug fixes.
@@ -45,6 +54,8 @@ There are no changes to the Opentrons App in v8.3.2, but it is required for upda
 
 The 8.3.1 hotfix release includes a small fix to allow all robots to properly reboot after an upgrade to v8.3.0.
 
+---
+
 ## Opentrons App Changes in 8.3.0
 
 Welcome to the v8.3.0 release of the Opentrons App! This release adds support for Mandarin in the app or Flex touchscreen and includes other beta features for our commercial partners.