Further improve NPC trade labels and comments

[SatoMew]

This PR partially amends #384 by correcting the information about the previously mentioned localization oversight and reworks the documentation of InGameTrade_CheckForTradeEvo a bit.

Should evolve_trade.asm be renamed to avoid confusion with EVOLVE_TRADE since the file is only for InGameTrade_CheckForTradeEvo?

[Narishma-gb]

Slightly off-topic, would you consider using constants in in_game_trades.asm ?
Trade text dialogs are indexed at wInGameTradeTextPointerTableIndex, and are used as raw numbers in DoInGameTradeDialogue.

; in game trade text pointers
; TradeTextPointersX indexes (see engine/events/in_game_trades.asm)
	const_def
	const TEXT_WANNA_TRADE ; 0
	const TEXT_NO_TRADE    ; 1
	const TEXT_WRONG_MON   ; 2
	const TEXT_THANKS      ; 3
	const TEXT_AFTER_TRADE ; 4

I used these constant names in Pokegreen, see https://github.com/Narishma-gb/pokegreen/blob/24ad50fa9c7295e7b2fd003a7b40386cc02a7d32/engine/events/in_game_trades.asm