Bluetooth: Improve comments on the HCI_Delete_Store_Link_Key issue

Some Bluetooth controllers doesn't support this command so we first
need to check for its support before sending it. This patch adds a
lengthful commentary about this.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>