Update the bot PR description for updating lockfiles (#3910)

## Motivation and Context
#3907 revealed that bot-generated PR description was not accurate. This
PR improves the description to provide clearer guidance.

## Testing
Verified that the new description was rendered as expected (as shown in
[this dummy PR](#3909))

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

.github/workflows/pull-request-updating-lockfiles.yml

@@ -113,9 +113,10 @@ jobs:
       env:
         GITHUB_TOKEN: ${{ secrets.RELEASE_AUTOMATION_BOT_PAT }}
       run: |
+        echo -e 'If CI fails, commit the necessary fixes to this PR until all checks pass.\n\nIf changes are required to [crateNameToLastKnownWorkingVersions](https://github.com/smithy-lang/smithy-rs/blob/92916b5484cdfef9ff58540ebf5e845eeeccf860/aws/sdk/build.gradle.kts#L504), revert the first commit in the PR, run `./gradlew aws:sdk:cargoUpdateAllLockfiles`, and commit the updated lockfiles.' | \
         gh pr create \
           --title 'Run `cargo update` on the runtime lockfiles and the SDK lockfile' \
-          --body 'If CI fails, commit the necessary fixes to this PR until all checks pass. If required, update entries in [crateNameToLastKnownWorkingVersions](https://github.com/smithy-lang/smithy-rs/blob/6b42eb5ca00a2dc9c46562452e495a2ec2e43d0f/aws/sdk/build.gradle.kts#L503-L504).' \
           --base ${{ inputs.base_branch }} \
           --head ${{ steps.branch-name-for-updating-lockfiles.outputs.branch_name }} \
-          --label "needs-sdk-review"
+          --label "needs-sdk-review" \
+          --body-file -