docs: if you add_column, then you have to backfill instead of refresh

[dantasse]

Fixes https://linear.app/lancedb/issue/GEN-432/docs-update-udtf-doc-so-add-columns-does-a-backfill-not-a-refresh

It's not 100% ideal, because if you do:

clips = db.create_materialized_view(
    "clips",
    query=videos.search(None).select(["video_path", "metadata"]),
    udtf=extract_clips,
)
clips.refresh()
clips.add_columns({"embedding": clip_embedding})
clips.backfill("embedding")

then later add more rows to videos and run clips.refresh(), clips will have clips from the newly added videos, but will not have embedding filled in, and people might expect that they would. But I think it's comprehensible enough - like "refresh()" can't "just magically fix everything in the downstream table."

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[dantasse]

Oh, doesn't require a review - I'll just merge it. Lmk all if this is not what you want in the future.