Add note in docs for array slices that output is always centered on the Yee cell

[oskooi]

In the documentation for output_dft, it is mentioned that:

Note that although the various field components are stored at different places in
the Yee lattice, when they are outputted they are all linearly interpolated to the 
same grid: to the points at the centers of the Yee cells, i.e. (i+0.5,j+0.5,k+0.5)⋅Δ
in 3d.

However, there is no such description for the array-slice routines get_array and get_dft_array. This interpolation feature is an important part of Meep's design philosophy of ensuring (for the user) an illusion of continuity.