Document limitation: cannot flatten OR/AND chains in EXPLAIN output

The 02711_trim_aliases test expects OR chains like "a OR b OR c" to be
flattened to or(a, b, c) with 3 children. However, tests like 01661_join_complex
expect "(a OR b) OR c" to remain nested as or(or(a,b), c) with 2 children.

Both SQL patterns produce identical parse trees due to left-associative parsing.
Our parser doesn't track explicit parenthesization, but ClickHouse's EXPLAIN
output distinguishes between them based on the original SQL syntax.

Since we cannot tell implicit grouping apart from explicit parentheses at the
AST level, any fix for 02711 would break 01661 (and vice versa).

The 02711_trim_aliases test remains in explain_todo as this is a fundamental
limitation of our parser architecture.

Author: claude

internal/explain/expressions.go

@@ -264,6 +264,11 @@ func explainBinaryExpr(sb *strings.Builder, n *ast.BinaryExpr, indent string, de
 		return
 	}
 
+	// Note: OR and AND chains are NOT flattened because we cannot distinguish
+	// implicit left-associativity from explicit parenthesization in the AST.
+	// For example: "a OR b OR c" and "(a OR b) OR c" produce identical parse trees
+	// but ClickHouse EXPLAIN outputs different structures for them.
+
 	fmt.Fprintf(sb, "%sFunction %s (children %d)\n", indent, fnName, 1)
 	fmt.Fprintf(sb, "%s ExpressionList (children %d)\n", indent, 2)
 	Node(sb, n.Left, depth+2)