Update help text on Automated list

Problem/Motivation

The help text in the Limit accordion on the Automated list is confusing. And, using 0 when the list is set to Unlimited means the default number of 12 cards will display, this is not indicated in the help text.

Steps to reproduce

1. Create an automated list
2. In the Content tab, open the Limit panel and read the help text for the Limit type and the Limit fields

Observed

• Limit type: Use Unlimited to show all results split into pages or Limited to reduce number of results to a specified Limit.
• Limit: Number of items to display if Limit type is Limited or items per page if Limit type is Unlimited.
  Use 0 to show all items.

Acceptance criteria

GIVEN I’m a Content author
WHEN I read the help text on the Automated list Limit accordion
THEN it as as follows:

• Limit type: Use Limited to show a limited number of results in the list, according to the number you add in the Limit field. Use Unlimited to show all results, split into pages using pagination.
• Limit: Select the number of items you want to display in the list if the Limit type is Limited, or the number of items per page if the Limit type is Unlimited. Using 0 will default to 12 items if the Limit type is Unlimited.