Clarify documentation for sprite property

[ryanhamley]

The style docs say that the sprite property is required if a layer uses the background-pattern, fill-pattern, fill-extrusion-pattern, line-pattern and icon-image properties. The standalone sprite section of the docs however doesn't mention the fill-extrusion-pattern requirement.

In either case, this is confusing. These properties may be necessary when setting up a map's root style, but at runtime you can use the addImage method to help setup a layer that makes use of the *-pattern and icon-image properties without requiring a sprite URL.

This came up as I was trying to add validation for styles that use the "required" properties without a sprite URL in #6831. A strict test that throws an error turns out to be a breaking change for styles that use these properties in runtime via addImage.

[jfirebaugh]

Aha -- yes, we even used to have the validation, but removed it with the addition of addImage.

f50a304#diff-d2fa2e781710b64955ee1cf6467242d4

#6231 is related. The ideal behavior in my opinion is to validate that image references are valid at the time they are added to the style.

[ansis]

The docs now mention fill-extrusion-pattern. It also no longer says that the sprite is required ("its icons can be used to render..."). The docs could be better (like all our style-spec docs could be) but these specific issues seem fixed.