Bluetooth: Host: Doc `bt_conn_foreach` includes disconnected

A user reported confusion about whether disconnected `bt_conn` are
meant to be found by `bt_conn_foreach`. The confusion likely stems from
the inprecise wording "existing connections".

This commit doucments `bt_conn_foreach` as it is: A low-level API for
iterating trough all `bt_conn` objects, connected or not.

This commit also includes a warning about possibly unintutive behavior
in preemptible threads.

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/53996



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