Sync migration docs with upload-artifact

We've updated them quite a bit in upload-artifact so let's make sure we're consistent.

docs/MIGRATION.md

@@ -2,6 +2,8 @@
 
 - [Migration](#migration)
   - [Multiple uploads to the same named Artifact](#multiple-uploads-to-the-same-named-artifact)
+  - [Overwriting an Artifact](#overwriting-an-artifact)
+  - [Merging multiple artifacts](#merging-multiple-artifacts)
 
 Several behavioral differences exist between Artifact actions `v3` and below vs `v4`. This document outlines common scenarios in `v3`, and how they would be handled in `v4`.
 
@@ -31,6 +33,7 @@ jobs:
       - name: Download All Artifacts
         uses: actions/download-artifact@v3
         with:
+          name: my-artifact
           path: my-artifact
       - run: ls -R my-artifact
 ```
@@ -71,6 +74,7 @@ jobs:
 -     uses: actions/download-artifact@v3
 +     uses: actions/download-artifact@v4
       with:
+-       name: my-artifact
         path: my-artifact
 +       pattern: my-artifact-*
 +       merge-multiple: true
@@ -78,3 +82,127 @@ jobs:
 ```
 
 In `v4`, the new `pattern:` input will filter the downloaded Artifacts to match the name specified. The new `merge-multiple:` input will support downloading multiple Artifacts to the same directory. If the files within the Artifacts have the same name, the last writer wins.
+
+## Overwriting an Artifact
+
+In `v3`, the contents of an Artifact were mutable so something like the following was possible:
+
+```yaml
+jobs:
+  upload:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Create a file
+        run: echo "hello world" > my-file.txt
+      - name: Upload Artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: my-artifact # NOTE: same artifact name
+          path: my-file.txt
+  upload-again:
+    needs: upload
+    runs-on: ubuntu-latest
+    steps:
+      - name: Create a different file
+        run: echo "goodbye world" > my-file.txt
+      - name: Upload Artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: my-artifact # NOTE: same artifact name
+          path: my-file.txt
+```
+
+The resulting `my-file.txt` in `my-artifact` will have "goodbye world" as the content.
+
+In `v4`, Artifacts are immutable unless deleted. To achieve this same behavior, you can use `overwrite: true` to delete the Artifact before a new one is created:
+
+```diff
+jobs:
+  upload:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Create a file
+        run: echo "hello world" > my-file.txt
+      - name: Upload Artifact
+-       uses: actions/upload-artifact@v3
++       uses: actions/upload-artifact@v4
+        with:
+          name: my-artifact # NOTE: same artifact name
+          path: my-file.txt
+  upload-again:
+    needs: upload
+    runs-on: ubuntu-latest
+    steps:
+      - name: Create a different file
+        run: echo "goodbye world" > my-file.txt
+      - name: Upload Artifact
+-       uses: actions/upload-artifact@v3
++       uses: actions/upload-artifact@v4
+        with:
+          name: my-artifact # NOTE: same artifact name
+          path: my-file.txt
++         overwrite: true
+```
+
+Note that this will create an _entirely_ new Artifact, with a different ID from the previous.
+
+## Merging multiple artifacts
+
+In `v3`, multiple uploads from multiple jobs could be done to the same Artifact. This would result in a single archive, which could be useful for sending to upstream systems outside of Actions via API or UI downloads.
+
+```yaml
+jobs:
+  upload:
+    strategy:
+      matrix:
+        runs-on: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.runs-on }}
+    steps:
+      - name: Create a File
+        run: echo "hello from ${{ matrix.runs-on }}" > file-${{ matrix.runs-on }}.txt
+      - name: Upload Artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: all-my-files # NOTE: same artifact name
+          path: file-${{ matrix.runs-on }}.txt
+```
+
+The single `all-my-files` artifact would contain the following:
+
+```
+.
+  ∟ file-ubuntu-latest.txt
+  ∟ file-macos-latest.txt
+  ∟ file-windows-latest.txt
+```
+
+To achieve the same in `v4` you can change it like so:
+
+```diff
+jobs:
+  upload:
+    strategy:
+      matrix:
+        runs-on: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.runs-on }}
+    steps:
+      - name: Create a File
+        run: echo "hello from ${{ matrix.runs-on }}" > file-${{ matrix.runs-on }}.txt
+      - name: Upload Artifact
+        uses: actions/upload-artifact@v3
+        with:
+-         name: all-my-files
++         name: my-artifact-${{ matrix.runs-on }}
+          path: file-${{ matrix.runs-on }}.txt
++ merge:
++   runs-on: ubuntu-latest
++   needs: upload
++   steps:
++     - name: Merge Artifacts
++       uses: actions/upload-artifact/merge@v4
++       with:
++         name: all-my-files
++         pattern: my-artifact-*
+```
+
+Note that this will download all artifacts to a temporary directory and reupload them as a single artifact. For more information on inputs and other use cases for `actions/upload-artifact/merge@v4`, see [the action documentation](../merge/README.md).