Minor rewording/tweaks

@timj - here are a couple of wording tweaks and acronym definitions the first time they are used. I also have a couple of other comments:

* I may have missed this, but I did not find a good discussion about missing values and how badly FITS supports them. FITS only supports defining missing values (NULL) for integer types, and I *think* only for tables. @embray can confirm. Of course, some people use NaN for missing values, but that is not correct - there is a difference between NaN (the result of a computation that produces an invalid result) and a completely missing data point. I think it would be worth adding a section about this. I could try and help with that, but maybe @embray would be better qualified to comment on it.

* Minor: You mention a couple of times the term 'dollars' - you may want to specify US dollars (and if at all possible, word it in a different way to avoid having to use it at all, since who knows what the USD will be worth five years from now ;)

I would be very happy to join the paper as an author - should I add this to the present PR?