Extend jerry_create_realm API docs a bit (#4549)

Add additional information for jerry_create_realm method and rework the
example to be a compilable/runnable doctest.

JerryScript-DCO-1.0-Signed-off-by: Peter Gal pgal.usz@partner.samsung.com

docs/02.API-REFERENCE.md

@@ -6131,6 +6131,14 @@ jerry_create_undefined (void);
 
 Creates a `jerry_value_t` representing a new global object.
 
+*Notes*:
+- Returned value must be freed with [jerry_release_value](#jerry_release_value)
+  when it is no longer needed.
+- This API depends on a build option (`JERRY_BUILTIN_REALMS`) and can be checked
+  in runtime with the `JERRY_FEATURE_REALM` feature enum value.
+  see: [jerry_is_feature_enabled](#jerry_is_feature_enabled).
+- The es.next profile enables this by default.
+
 **Prototype**
 
 ```c
@@ -6144,13 +6152,24 @@ jerry_create_realm (void);
 
 **Example**
 
+[doctest]: # ()
+
 ```c
+#include "jerryscript.h"
+
+int
+main (void)
 {
+  jerry_init (JERRY_INIT_EMPTY);
+
   jerry_value_t realm_value = jerry_create_realm ();
 
-  ... // usage of the value
+  // usage of the value
 
   jerry_release_value (realm_value);
+  jerry_cleanup ();
+
+  return 0;
 }
 ```