docs: clarify type system characteristics in types.md

Update the documentation to specify that explicit casting is required between different types, with the exception of conversions between primitive reference types and their corresponding fundamentals. Adjust code examples to reflect this change, enhancing clarity and understanding of the type system.

Author: bialger

docs/reference/types.md

@@ -220,7 +220,7 @@ fun CanReassign(var str: String): Void {
 ## Type System Characteristics
 
 **Static typing:** Every variable and expression has a type checked at compile time
-**No implicit conversions:** Explicit casting required between different types
+**No implicit conversions except builtin wrappers:** Explicit casting required between different types, except for conversions between primitive reference types and their corresponding fundamentals.
 **Type safety:** Prevents many common errors
 **Nullable types:** Any reference type (including primitive reference types) can be made nullable by appending `?`. Fundamental types cannot be nullable.
 
@@ -236,7 +236,7 @@ val result: float = (intVal as float) + floatVal  // OK: Explicit conversion
 // Using primitive reference types (implicit conversion)
 val refInt: Int = 42  // Implicit conversion from literal
 val refFloat: Float = 3.14  // Implicit conversion from literal
-val sum: Int = refInt + refFloat  // Implicit: Int + Float -> Int
+val sum: Int = refInt + (refFloat as Int)  // Explicit conversion
 val fundamentalSum: int = sum + 10  // Implicit: Int + int -> int
 
 // Converting nullable primitives to fundamentals