I'll explain why this matters. One of the important points of my rules document is to not introduce any rules that are unclear or open to interpretation. Since the aim is that the reader should be able to determine any interaction, this is important.
Sure, some cards introduce exceptions to the rules. For instance, it's enough in the main rules explanation to say that you buy from the Supply. You don't have to say that Black Market provides an exception; this is explained under the entry for Black Market (in addition to the card text itself allowing it).
But as a counterexample, take Crown being played at start of turn. It matters for Crown whether it's your Buy phase, your Action phase, or neither. So you shouldn't say that the Action phase starts after the start-of-turn abilities, and then explain under the entry for Crown that it plays an Action card anyway (as a special ruling about Crown). You should state that first the Action phase starts, then you do start-of-turn abilities, then you may play Action cards by using Actions. Both approaches would make the reader understand how Crown works at start of turn. But the first approach makes it confusing what happens when you play Crown in another phase (Clean-up or Night) or on another player's turn, all of which are possible now. It's actually the wrong explanation too, because Donald has confirmed that it's your Action phase at start of turn.
The rulebooks state that you can only buy a card that you can afford. This is the rule I have in my document. Up until now, it says that the first step in buying a card is choosing any visible card in the Supply that you can afford. My original question was because of this: should this explanation be changed? My conclusion is that it shouldn't. Animal Fair creates its own exception, it implicitly says that you can choose it even though you can't afford its cost because you can choose to do something else instead of paying that cost.