Make spacingScale description more accurate in theme.json schema

[t-hamano]

What?

This PR updates the description of spacingScale introduced by #41527

Why?

In the input support by JSON schema, the descriptions of spacingSizes and spacingScale are exactly the same, so it may be difficult to understand the difference between them.

spacingScale

spacingSizes

Furthermore, spacingScale doesn'ot generate presets by itself, but is a property that defines the logic for generating presets.

How?

I have updated the description to what I think is appropriate according to what this property originally meant.

Testing Instructions

Create a JSON file that references the JSON schema for this branch, such as:

{
	"$schema": "https://raw.githubusercontent.com/WordPress/gutenberg/schema/theme-json-spacing-scale/schemas/json/theme.json",
	"version": 2,
	"settings": {
	}
}

In settings.spacingScale, confirm that the new description is proposed.

[ajlende]

Thanks for the change! spacingScale is more clear now