tweaks to country, datetime, and generic field copy

resources/automagic_dashboards/field/Country.yaml

 title: A look at your [[this]]
 transient_title: Here's a closer look at your [[this]] field
-description: How [[GenericTable]] are distributed across [[this]], and how each country is represented in different categories.
+description: The number of [[GenericTable]] per country, and how each country is represented in different categories.
 applies_to: GenericTable.Country
 metrics:
 - Sum: [sum, [dimension, GenericNumber]]

resources/automagic_dashboards/field/DateTime.yaml

 title: A look at your [[this]]
 transient_title: Here's a closer look at your [[this]]
-description: See how [[GenericTable]] are distributed across this field, and if it has any seasonal patterns.
+description: How [[GenericTable]] are distributed across this time field, and if it has any seasonal patterns.
 applies_to: GenericTable.DateTime
 metrics:
 - Sum: [sum, [dimension, GenericNumber]]

resources/automagic_dashboards/field/GenericField.yaml

 title: A look at your [[this]]
 transient_title: Here's an overview of your [[this]]
-description: How this field is distributed, how many distinct values it has, and how it changes over time.
+description: How [[GenericTable]] are distributed across this field, and how it changes over time.
 applies_to: GenericTable.*
 metrics:
 - Sum: [sum, [dimension, GenericNumber]]