storage/index.md: bind mounts must use absolute paths

[RobBednark]

Proposed changes

Clarified that bind mounts must use absolute paths, else they will silently "fail" (have unexpected behavior). The language in the existing docs is not strong enough.

Related issues (optional)

Related to:
moby/moby#4830 (comment)
docker/cli#1203

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[docker-robott]

Thanks for the pull request. We'd like to make our product docs better, but haven’t been able to review all the suggestions.
As our docs have also diverged, we do not have the bandwidth to review and rebase old pull requests.

If the updates are still relevant, review our contribution guidelines and rebase your pull request against the latest version of the docs, then mark it as fresh with a /remove-lifecycle stale comment.
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