Change posterity notation

[someone235]

No description provided.

[freshair18]

While on the subject of revising this file I wanna point out another minor issue:

Regarding proof of publication, it currently says "Merkle proof that txn is in C's ATMR". The ATMR does not actually contain the transactions of the corresponding block, rather only the accepted txs of its mergeset. These makes for a subtler concept where it is proven that a tx was accepted according to some block at some point, but maybe not according to the selected chain.
Maybe this concept has merits of itself but I don't see them as of now and surely the intention was not for it - so it should be changed to "merkle proof that txn is in C's block transactions" or something of this line, and the section on atmr should also change accordingly.

Also, in the code I currently use the terms post_posterity and pre_posterity rather than prev and next. Not crucial but its nice if there is consistency between code and kip. I prefer mine because I always find the word next a tad ambiguous.