Document prepare_timeout setting

Daniel Keenanrequested to merge
39375-document-prepare-stage-timeout into main

What does this MR do?

Documents the prepare_timeout setting.

Why was this MR needed?

Part of #26583 (closed)

What's the best way to test this MR?

Inspect the docs.

What are the relevant issue numbers?

Closes #39375 (closed)

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