Confusion of HPX projection vs HEALPix in general

Noticed by DSB.
Explain why nested is convenient with HPX

Requested by DSB.
Move S-2 calib. reference to constant FCF bit

"should be a constant" is exactly what JTD's paper says about
the FCF, so just move the citation marker to this phrase in order
to answer DSB's query about whether the FCF really is constant.
We could put in the 5% and 10% numbers from the paper here but
that probably isn't necessary.