Add explanations to battle summaries

[bglw]

Of the form: "X beat Y because ...", to help explain the rules as the game is played.

i.e.

• "RAIDER was a valid word, beating GAMORN which was not a valid word"
• "MANE beats TA on length due to being ≥2 letters longer"

Plus explanations for larger/complex battles like OWING vs BET & ESTATE or AD & DOMINION vs YELLOW