docs(storage-ktx): document kotlin destructuring declarations

[thatfiredev]

DO NOT MERGE before the next release of storage-ktx.

This should document the new features added in #1644

[google-oss-bot]

Hi @rosariopfernandes. Thanks for your PR.

I'm waiting for a firebase member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[thatfiredev]

KTX docs were removed from firebaseopensource.com in #2363, so I think this PR is no longer needed.