Map Block: improve initial placeholder text to avoid folks setting up restricted access tokens

When setting up a Map Block, you are presented with this screen:


If you use the default public access token, you'll be all set and things will work well.

However, you may be tempted to set up a new access token with URL restrictions, and restrict access to your own site. That won't work as such restricted tokens cannot be used for map embeds like the ones we use:

> Use a public access token (pk.*) with Mapbox GL, not a secret access token (sk.*). See https://www.mapbox.com/api-documentation/#access-tokens-and-token-scopes

To make sure no one makes that error, we may be able to update the wording on that initial screen to make things clearer.

Start at the Map Block's initial setup screen described in the issue and review the existing placeholder wording. Update the text to explain that a public access token is required and that URL-restricted tokens will not work for these embeds. Confirm the screen clearly guides users away from creating restricted tokens.

Written by the indexing model from the issue text.