Clarify which properties can use exponential and interval functions in docs

[samanpwbb]

Small fix to docs, @jfirebaugh for review. I went with "requires a property that can be interpolated" instead of "requires an interpolatable property" because "interpolated" is a keyword from the spec JSON that people might be searching for on the page.

[jfirebaugh]

Ah shoot, I misunderstood your question and misread the docs. "The domain must be numeric" is actually correct. Note: domain, not range. Both interval and exponential functions require numeric domain values -- they are either zoom functions, or property or zoom-and-property functions with numeric property values.

[jfirebaugh]

We should say something about the range/output type though.

For exponential:

The domain (input value) must be numeric, and the style property must support interpolation. Style properties that support interpolation are marked with ◢, and exponential is the default function type for these properties.

For interval:

The domain (input value) must be numeric. Any style property may use interval functions. For properties marked with 📶, the "interval" symbol, this is the default function type.