tigris: document shadow write-through preservation on update

[tripledoublev]

Summary

Documents that on fly storage update, shadow attributes must all be resupplied, and omitting --shadow-write-through preserves the bucket's current write-through setting instead of resetting it to disabled.

Blocked — do not merge yet

Describes behavior from superfly/flyctl#5119, which hasn't merged. Today, supplying the shadow attributes without --shadow-write-through resets it to disabled. Keep as draft until #5119 is merged and released.

Ported to tigris/index.mdx after the Mintlify cutover; the rest of the original change landed via #2469.

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[kcmartin]

Heads up: the docs move to Mintlify next Wednesday, so this file becomes tigris/index.mdx and this PR won't merge cleanly afterwards. Since it's blocked on superfly/flyctl#5119 anyway, I'll carry the change into the new file once that lands and credit you. Nothing needed from you.

[kcmartin]

This one is stuck. It edits tigris/index.html.markerb, which we deleted at the Mintlify cutover. The page is tigris/index.mdx now.

Happy to port the change over if you'd rather not redo it.

Tracking the stranded PRs in superfly/docs-tracking#314.

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[tripledoublev]

ported it to tigris/index.mdx. you can take it off docs-tracking#314. keeping it draft until flyctl#5119 ships, since it documents that fix