Bluetooth: host: Document security changed and no security level change

Document the security changed callback can be called with no change
in the security level when the encryption key has been changed
in the link-layer.

Also update the documentation for set security function to include
the bond information so that it matches the documentation for security
changed callback.

Update pairing process to pairing procedure for consistency.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>