checker: detect recursive .str() method calls (#27926)

Author: medvednikov

doc/docs.md

@@ -3600,6 +3600,23 @@ red := Color{
 println(red)
 ```
 
+**Note:** Calling `.str()` on an unchanged receiver, or an unchanged alias of it,
+inside a custom `str()` method is not allowed because it causes infinite recursion.
+Use string interpolation of the individual fields instead, or for type aliases,
+cast to the underlying type first:
+
+```v failcompile
+struct Color {
+	r int
+	g int
+	b int
+}
+
+fn (c Color) str() string {
+	return c.str() // error: cannot call `str()` method recursively
+}
+```
+
 ### Dumping expressions at runtime
 
 You can dump/trace the value of any V expression using `dump(expr)`.