Bluetooth: Host: Document `bt_set_name` kconfig requirements

Without `CONFIG_BT_DEVICE_NAME_DYNAMIC`, `bt_set_name` always return
`-ENOMEM`. Users looking to use `bt_set_name` should be pointed to the
relevant kconfig that enables its useful operation.

Also add a "see also" pointer to `CONFIG_BT_DEVICE_NAME_MAX`, which
controls the maximum length of the name passed to `bt_set_name`.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>