Update string.h.md

doc/api/mruby/string.h.md

@@ -8,3 +8,8 @@ Adds to strings together.
    mrb_value mrb_ptr_to_str(mrb_state *, void*);
 ```
 Converts pointer into a Ruby string.
+### mrb_obj_as_string
+```C
+   mrb_obj_as_string(mrb_state *mrb, mrb_value obj);
+```
+Returns an object as a Ruby string.