bluetooth: fast_pair: fmdn: improve callback API doc

Improved the documentation for the callback registrations APIs in the
Fast Pair FMDN header.

Ref: NCSDK-30487

Signed-off-by: Kamil Piszczek <[email protected]>

include/bluetooth/services/fast_pair/fmdn.h

@@ -234,6 +234,9 @@ struct bt_fast_pair_fmdn_ring_cb {
  *  You can call this function only in the disabled state of the FMDN module
  *  (see @ref bt_fast_pair_is_ready function).
  *
+ *  This function must be called in the cooperative thread context or in the system initialization
+ *  context (@ref SYS_INIT macro).
+ *
  *  @param cb Ringing callback structure.
  *
  *  @return 0 if the operation was successful. Otherwise, a (negative) error code is returned.
@@ -355,6 +358,9 @@ struct bt_fast_pair_fmdn_motion_detector_cb {
  *  You can call this function only in the disabled state of the FMDN module
  *  (see @ref bt_fast_pair_is_ready function).
  *
+ *  This function must be called in the cooperative thread context or in the system initialization
+ *  context (@ref SYS_INIT macro).
+ *
  *  @param cb Motion detector callback structure.
  *
  *  @return 0 if the operation was successful. Otherwise, a (negative) error code is returned.
@@ -529,6 +535,9 @@ struct bt_fast_pair_fmdn_info_cb {
  *  This API for callback registration is optional and does not have to be used. You can
  *  register multiple instances of information callbacks.
  *
+ *  This function must be called in the cooperative thread context or in the system initialization
+ *  context (@ref SYS_INIT macro).
+ *
  *  @param cb Information callback structure.
  *
  *  @return 0 if the operation was successful. Otherwise, a (negative) error code is returned.
@@ -565,6 +574,9 @@ struct bt_fast_pair_fmdn_read_mode_cb {
  *  You can call this function only in the disabled state of the FMDN module
  *  (see @ref bt_fast_pair_is_ready function).
  *
+ *  This function must be called in the cooperative thread context or in the system initialization
+ *  context (@ref SYS_INIT macro).
+ *
  *  @param cb Read mode callback structure.
  *
  *  @return 0 if the operation was successful. Otherwise, a (negative) error code is returned.