Bluetooth: host: Kconfig: Correct help of BT_BONDABLE

Correct the `help` of the configuration `BT_BONDABLE`.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>

subsys/bluetooth/host/Kconfig

@@ -632,9 +632,13 @@ config BT_BONDABLE
  bool "Bondable Mode"
  default y
  help
- This option enables support for Bondable Mode. In this mode,
- Bonding flag in AuthReq of SMP Pairing Request/Response will be set
- indicating the support for this mode.
+ This option is the default value of the bonding flag for any ACL connection.
+ If the option is true, the default bonding flag is true. Or, the default
+ bonding flag is false.
+ After a connection is established, the bonding flag of the connection
+ can also be changed by calling `bt_conn_set_bondable()` if the configuration
+ `the bonding flag per-connection` (BT_BONDABLE_PER_CONNECTION) is
+ enabled. Please see the BT_BONDABLE_PER_CONNECTION configuration.
 
 config BT_BONDING_REQUIRED
  bool "Always require bonding"