Bluetooth: Mesh: Update Kconfig help about Mesh on System WQ

This updates the help text for the Kconfig option BT_MESH_WORKQ_SYS to
take into account the change made in PR #84282 which causes the host to
no longer return -ENOBUFS. Since the mesh will now instead block the
work queue, a note has been added about a potential consequence of this.

Signed-off-by: Ludvig Jordet <ludvig.jordet@nordicsemi.no>