Add a note for multible changes in one file (#37090)

As mentioned in this answer https://stackoverflow.com/a/41549694/6778826 we was able to solve a bad behaviour of `blockinfile` module.
We must change two parts in one file which we have done in two tasks. Every run have rewriten the first block with the second block in the wrong position. **After** we have set the `marker` in the second task to another value was the `blockinfile` module able to insert both parts in the right position in the same file.

lib/ansible/modules/files/blockinfile.py

@@ -95,6 +95,7 @@ notes:
   - When using 'with_*' loops be aware that if you do not set a unique mark the block will be overwritten on each iteration.
   - As of Ansible 2.3, the I(dest) option has been changed to I(path) as default, but I(dest) still works as well.
   - Option I(follow) has been removed in version 2.5, because this module modifies the contents of the file so I(follow=no) doesn't make sense.
+  - When more then one block should be handled in **one** file you **must** change the I(marker) per task
 """
 
 EXAMPLES = r"""