provider:azurerm: Add documentation for `azurerm_virtual_machine` adminPassword
provider:azurerm: Add documentation for `azurerm_virtual_machine`
adminPassword

Reports from issues showed the following errors:

```
{
    "error": {
            "code": "InvalidParameter",
                    "target": "adminPassword",
                            "message": "The supplied password must be
                            between 6-72 characters long and must
                            satisfy at least 3 of password complexity
                            requirements from the following: \r\n1)
                            Contains an uppercase character\r\n2)
                            Contains a lowercase character\r\n3)
                            Contains a numeric digit\r\n4) Contains a
                            special character."

    }

}
```

This commit adds some documentation for the adminPassword complexity
requirements

thiagocaiubi/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm,sebastus/terraform-provider-azurerm,sebastus/terraform-provider-azurerm,thiagocaiubi/terraform-provider-azurerm,thiagocaiubi/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm,sebastus/terraform-provider-azurerm,abhijeetgaiha/terraform-provider-azurerm