Update transport version documentation (elastic#138263)

This adds additional information about cherry-picking an original 
change when a decision has been made to add a backport version.

Co-authored-by: Ryan Ernst <[email protected]>

Author: 2 people

docs/internal/Versioning.md

@@ -110,8 +110,34 @@ needed to backport to `8.19` you would run (in `main`):
     ./gradlew generateTransportVersion --name=my_tv --backport-branches=9.1,8.19
 
 In the above case CI will not know what transport version name to update, so you
-must run the generate task again as described. After merging the updated transport
-version it will need to be backported to all the applicable branches.
+must run the generate task again as described. After merging the updated
+transport version it will need to be backported to all the applicable branches.
+Backport branches that already included the original change can use
+auto-backport in CI or cherry-pick the updated transport version. Backport
+branches that did not include the original change need to cherry-pick both the
+original change and the updated transport version. For example, with the above
+change that is first backported to `9.1`, and subsequently added to `8.19`, the
+sequence of events would look like the following:
+
+1. Create a pull request with target branch `main` for a new change
+(`C0`) with a newly created transport version (`TV`) required by `C0`
+   1. Use the `github` labels `auto-backport` and `branch:9.1` to backport `C0`
+   and `TV` into the target branch `9.1`.
+2. Discover that `C0` is also required in target branch `8.19`.
+3. Create a pull request with target branch `main` for a change (`C1`) to only
+update `TV` to include `8.19`
+   1. Use the gradle task
+   `./gradlew generateTransportVersion --name=my_tv
+   --backport-branches=9.1,8.19`
+   to update `TV` to include `8.19`
+   2. Use the `github` labels `auto-backport`, `branch:9.1`, and `branch:8.19`
+   to backport `C1` and updated `TV` into the target branches `9.1` and `8.19`.
+   This creates a source branch for `8.19` (`SB819`) that does not contain `C0`.
+   3. Cherry-pick `C0` into the source branch `SB819`
+
+Note there are several ways to achieve the desired end state. This
+specific example takes advantage of the available CI tooling to simplify the
+process.
 
 ### Resolving merge conflicts