feat: Mention that ~ can be used without username to access your own stores (#678)

Wanted to link to this in the docs and found out it's missing.

---------

Co-authored-by: Misha <m.murasovs@gmail.com>

Author: valekjo

sources/platform/storage/index.md

@@ -130,7 +130,8 @@ The [Apify API](/api/v2#/reference/key-value-stores) allows you to access your s
 In most cases, when accessing your storages via API, you will need to provide a **store ID**, which you can do in the following formats:
 
 * **WkzbQMuFYuamGv3YF** - the store's alphanumerical ID if the store is unnamed.
-* **username~store-name** - your username and the store's name separated by a tilde (`~`) character (e.g. **janedoe~ecommerce-scraping-results**) if the store is named.
+* **~store-name** - the store's name prefixed with tilde (`~`) character if the store is named (e.g. **~ecommerce-scraping-results**)
+* **username~store-name** - username and the store's name separated by a tilde (`~`) character if the store is named and belongs to a different account (e.g. **janedoe~ecommerce-scraping-results**). Note that in this case, the store's owner needs to grant you access first.
 
 For read (GET) requests, it is enough to use a store's alphanumerical ID, since the ID is hard to guess and effectively serves as an authentication key.