doc: Update block volume usage

Signed-off-by: hamistao <[email protected]>

doc/explanation/storage.md

@@ -132,7 +132,7 @@ Storage volumes can be of the following types:
 
 `custom`
 : You can add one or more custom storage volumes to hold data that you want to store separately from your instances.
-  Custom storage volumes can be shared between instances, and they are retained until you delete them.
+  Custom storage volumes of content type `filesystem` or `iso` can be shared between instances, and they are retained until you delete them.
 
   You can also use custom storage volumes to hold your backups or images.
 
@@ -154,7 +154,8 @@ Each storage volume uses one of the following content types:
   You can create a custom storage volume of type `block` by using the `--type=block` flag.
 
   Custom storage volumes of content type `block` can only be attached to virtual machines.
-  They should not be shared between instances, because simultaneous access can lead to data corruption.
+  By default, they can only be attached to one instance at a time, because simultaneous access can lead to data corruption.
+  Sharing a custom storage volumes of content type `block` is made possible through the usage of the `security.shared` configuration key.
 
 `iso`
 : This content type is used for custom ISO volumes.

doc/howto/storage_volumes.md

@@ -53,6 +53,7 @@ The following restrictions apply:
 - To avoid data corruption, storage volumes of {ref}`content type <storage-content-types>` `block` should never be attached to more than one virtual machine at a time.
 - Storage volumes of {ref}`content type <storage-content-types>` `iso` are always read-only, and can therefore be attached to more than one virtual machine at a time without corrupting data.
 - File system storage volumes can't be attached to virtual machines while they're running.
+- Custom block storage volumes that don't have `security.shared` enabled cannot be attached to more than one instance at the same time and neither can be attached to profiles.
 
 For custom storage volumes with the content type `filesystem`, use the following command, where `<location>` is the path for accessing the storage volume inside the instance (for example, `/data`):