Clarify MinIO instructions

Fix #8090.

Author: astrojuanlu

docs/development/install.rst

@@ -62,13 +62,10 @@ Set up your environment
 
    * go to http://localhost:9000/ (MinIO S3 storage backend)
    * login as ``admin`` / ``password``
-   * click "..." next to the bucket name and then "Edit Policy"
-   * give "Read Only" access on all the buckets (``static`` and ``media``)
-
-   .. note::
-
-      ``media`` bucket may be created after the first build is finished.
-      You will need to repeat this step after that.
+   * click "..." next to the ``static`` bucket name and then "Edit Policy"
+   * leave "prefix" empty and click "Add" to give "Read Only" access on the ``static`` bucket
+   * click on the "+" icon on the bottom-right corner, then "Create bucket" with the name ``media``,
+     hit Enter on the keyboard, and repeat the operation above to give "Read Only" access to it
 
 #. go to http://community.dev.readthedocs.io to access your local instance of Read the Docs.