docs: Add math section to syntax reference + improve example comments

- five-syntax-en/ko: Add Math comparison table (Harbour RTL vs Go math)
- go_math_compare.prg: Detailed English comments explaining each section
- Example lists updated with go_math_compare.prg

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/five-syntax-en.md

@@ -384,6 +384,45 @@ dbTarget:Close()
                 20K concurrent goroutines, 5K random fuzz
 ```
 
+## Math: Harbour RTL + Go math Together
+
+Five gives you two math systems in one file:
+
+```prg
+IMPORT "math"
+
+PROCEDURE Main()
+   LOCAL nVal
+
+   // Harbour RTL — simple, no IMPORT needed
+   ? Abs(-42.5)              // 42.5
+   ? Sqrt(144)               // 12
+   ? Round(3.14159, 2)       // 3.14
+   ? Max(10, 20)             // 20
+
+   // Go math — complete, 60+ functions
+   ? math.Sin(math.Pi / 6)  // 0.5
+   ? math.Pow(2, 10)         // 1024
+   ? math.Hypot(3, 4)        // 5
+   ? math.Log2(1024)          // 10
+
+   // Combined — use both freely
+   nVal := (1 / Sqrt(2 * math.Pi)) * Exp(-0.5 * math.Pow(0, 2))
+   ? "Normal PDF at 0:", Round(nVal, 6)
+
+   RETURN
+```
+
+| | Harbour RTL | Go math |
+|---|---|---|
+| Functions | 9 (Abs, Sqrt, Round, Int, Max, Min, Log, Exp, Mod) | 60+ |
+| IMPORT needed | No | `IMPORT "math"` |
+| Best for | Business calculations | Scientific/financial |
+| Precision | Standard | IEEE 754 |
+| Trig/Hyperbolic | No | Sin, Cos, Tan, Sinh, Cosh, ... |
+| Constants | No | Pi, E, Phi, Ln2, Sqrt2 |
+| Special values | No | NaN, Inf, MaxFloat64 |
+
 ## Example Files
 
 | File | Description |
@@ -397,4 +436,5 @@ dbTarget:Close()
 | `examples/go_concurrent.prg` | Parallel data pipeline |
 | `examples/go_websocket.prg` | WebSocket chat server |
 | `examples/go_extensions.prg` | All 9 extension syntax demo |
+| `examples/go_math_compare.prg` | Harbour RTL vs Go math side-by-side |
 | `examples/godump_demo.prg` | HB_FUNC Go API |