In the datasheet of the nRF8001, they specify in the reference schematic (page 51) the LC circuit to use with the integrated DC/DC converter (see below).
Why is there a small inductor L5 in series with L4?
Is it related to the self resonant frequency of the inductors? That was my first thought, but the 10nF inductor specified in their integration note (Taiyo Yuden LBMF1608T100K) has a minimum self resonant frequency of 32MHz. The exact frequency of the nRF8001 buck converter is not specified, but it seems to be well above the frequency of any buck converters.