improve: StringBuilder initialization

Add detailed documentation for size_hint parameter

Author: bobzhang

builtin/stringbuilder_buffer.mbt

@@ -19,9 +19,19 @@ struct StringBuilder {
 }
 
 ///|
+/// Creates a new string builder with an optional initial capacity hint.
+///
+/// Parameters:
+///
+/// * `size_hint` : An optional initial capacity hint for the internal buffer. If
+/// less than 1, a minimum capacity of 1 is used. Defaults to 0. It is the size of bytes, 
+/// not the size of characters. `size_hint` may be ignored on some platforms, JS for example.
+///
+/// Returns a new `StringBuilder` instance with the specified initial capacity.
+///
 pub fn StringBuilder::new(size_hint~ : Int = 0) -> StringBuilder {
   let initial = if size_hint < 1 { 1 } else { size_hint }
-  let data = FixedArray::make(initial, Byte::default())
+  let data : FixedArray[Byte] = FixedArray::make(initial, 0)
   { data, len: 0 }
 }