allocf.c: update and enhance documentation in the comment

src/allocf.c

@@ -9,7 +9,17 @@
 /* This function serves as the default memory allocation function and accepts two arguments:
  *
  * - `p`: The previous pointer to the memory region. For memory allocation, this parameter is NULL.
- * - `size`: The new size of the memory region to be returned.
+ * - `size`: The new size of the memory region to be returned. If size is 0, the memory region will be freed.
+ *
+ * All memory allocation from the inside of mruby uses this function.
+ *
+ * If you want to use your own memory allocator, you have two options:
+ *
+ *  - provide your own version of malloc() / realloc() / free()
+ *
+ *  - redefine mrb_basic_alloc_func() in your application.
+ *
+ * See doc/guides/memory.md for detail.
  */
 
 void*
@@ -21,7 +31,7 @@ mrb_basic_alloc_func(void *p, size_t size)
     return NULL;
   }
   else {
-    /* `ralloc(NULL, size)` works as `malloc(size)` */
+    /* `realloc(NULL, size)` should work as `malloc(size)` */
     return realloc(p, size);
   }
 }