docs: clarify that store: false fields are not persisted after a restart

[tharropoulos]

Change Summary

• rewrite the store row in the collections field table to say the value is removed before it is written to disk and is absent after a restart
• add a warning callout below the table explaining store: false values live only in-memory and vanish on restart, snapshot restore, node replacement, or upgrade
• caution against store: false for data needed after a restart, using a float[] vector field losing its vectors as the example
• add a tip to the image-search guide noting the discarded image is fine because its embedding field is stored by default, cross-linking the collections note

PR Checklist

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