Revise the API ArrayBuffer related operations (#4284)

- External ArrayBuffer construction with 0 length should be equivalent to `new ArrayBuffer(0)`
- Internally allocated ArrayBuffers should be detachable
- Externally allocated ArrayBuffers free callback should be called when underlying buffer is detached

JerryScript-DCO-1.0-Signed-off-by: Robert Fancsik frobert@inf.u-szeged.hu

docs/02.API-REFERENCE.md

@@ -9011,7 +9011,9 @@ jerry_get_arraybuffer_pointer (const jerry_value_t value);
 - `value` - Array Buffer object.
 - return value
   - pointer to the Array Buffer's data area.
-  - NULL if the `value` is not an Array Buffer object.
+  - NULL if the `value` is:
+    - not an ArrayBuffer object
+    - an external ArrayBuffer has been detached
 
 *New in version 2.0*.
 
@@ -9096,6 +9098,8 @@ jerry_value_t
 jerry_detach_arraybuffer (const jerry_value_t value);
 ```
 
+*Note*: If the ArrayBuffer has been created with `jerry_create_arraybuffer_external` the optional free callback is called on a successful detach operation
+
 - `value` - ArrayBuffer to be detached
 - return
   - null value if success