[Documentation]: Clarify how companion photometry measurements work, absolute photometry vs relative photometry to the host star

[mperrin]

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From a discussion between @mperrin and @AarynnCarter...

for space data, we ought to be able to do absolute photometry of companions, without relying on any model of the host star brightness. This would just rely on the absolute flux.

@AarynnCarter will dig into the code to refresh our collective memory of exactly how this is working under the hood. And we will document it clearly how it works.

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