Update docs to mention list_storage_passwords capability

As we found out after some digging around - users that need to run the | virustotal command need to have a role with the list_storage_passwords capability (or alternatively admin_all_objects, which is broader). Without this, the search will error out, citing

Error in 'virustotal' command: External search command exited unexpectedly with non-zero error code 1

The admin role by default has the required capability, but the power user does not. You might wanna add this to the documentation, or alternatively throw a more helpful error. ;)