Table subtitle doesn't describe average and percent columns

[MaxGhenis]

The subtitle on the table reads:

All monetary values are in billions. All non-monetary values are in millions.

This does not correctly describe percentage/share columns, or average columns:

• Percent with Tax Cut
• Percent with Tax Increase
• Average Tax Change
• Share of Overall Change
• pc_aftertaxinc

I'd suggest rephrasing this to something like:

All monetary totals are in billions. All counts are in millions. Averages and shares are as shown.

[andersonfrailey]

I like this idea, @MaxGhenis. I'll get a PR for it up.