Fix broken "Cell Output Must Be at the Top Level" example in notebook patterns

The previous "FIXED" example still had mo.md() called as a side effect
inside if/else blocks (never rendered) while mixing it with a DataFrame
in the same cell. Replace with the correct pattern: split into separate
cells where each displays exactly one thing at the top level.

docs/notebook-patterns.md

@@ -83,7 +83,7 @@ def fetch_details(client, DATAINDEX, results):
 
 Marimo only renders the **last expression at the top level** of a cell as rich output. An expression buried inside an `if`/`else`, `for`, `try`, or any other block is **not** displayed — it's silently discarded.
 
-**BROKEN** — `_df` inside the `if` branch is never rendered:
+**BROKEN** — `_df` inside the `if` branch is never rendered, and `mo.md()` inside `if`/`else` is also discarded:
 
 ```python
 @app.cell
@@ -93,26 +93,35 @@ def show_results(results, mo):
         mo.md(f"**Found {len(results)} results**")
         _df  # Inside an if block — marimo does NOT display this
     else:
-        mo.md("**No results found**")
+        mo.md("**No results found**")  # Also inside a block — NOT displayed
     return
 ```
 
-**FIXED** — assign inside the branches, display at the top level:
+**FIXED** — split into separate cells. Each cell displays exactly **one thing** at the top level:
 
 ```python
+# Cell 1: build the data, return it
 @app.cell
-def show_results(results, mo):
-    _output = None
-    if results:
-        _output = pl.DataFrame(results)
-        mo.md(f"**Found {len(results)} results**")
-    else:
-        mo.md("**No results found**")
-    _output  # Top-level last expression — marimo renders this
-    return
+def build_results(results, pl):
+    results_df = pl.DataFrame(results) if results else None
+    return (results_df,)
+
+# Cell 2: heading — mo.md() is the top-level expression (use ternary for conditional text)
+@app.cell
+def show_results_heading(results_df, mo):
+    mo.md(f"**Found {len(results_df)} results**" if results_df is not None else "**No results found**")
+
+# Cell 3: table — DataFrame is the top-level expression
+@app.cell
+def show_results_table(results_df):
+    results_df  # Top-level expression — marimo renders this as interactive table
 ```
 
-**Rule of thumb:** initialize a `_output = None` variable before any conditional, assign the displayable value inside the branches, then put `_output` as the last top-level expression. When it's `None` (e.g., the `else` path), marimo shows nothing — which is fine since the `mo.md()` already provides feedback.
+**Rules:**
+- Each cell should display **one thing** — either `mo.md()` OR a DataFrame, never both
+- `mo.md()` must be a **top-level expression**, not inside `if`/`else`/`for`/`try` blocks
+- Build conditional text using variables or ternary expressions, then call `mo.md(_text)` at the top level
+- For DataFrames, use a standalone display cell: `def show_table(df): df`
 
 ### Async Cells