Fix language drift and connect "property functions" to data-driven styles

[tmcw]

Via a support question this morning:

Can I get a list of which layout/paint properties are supported for use with Property Functions... The documentation just says it is not fully supported and I can't easily tell from the code.

Right now we have, under Property Functions:

Note that support for property functions is not available across all properties and platforms at this time.

But then in the per-property spec

We need to connect these things by

1. Explicitly telling people how to check if a thing is supported
2. OR Using the same words to describe "property functions" and "data driven styles"

[jfirebaugh]

"data-driven styles" / "data-driven styling" is an umbrella term. "property functions" are one type of data-driven function. "zoom-and-property functions" is another. (Arguably, the interpolation syntax supported by text-field and icon-image are a third.)

The documentation should refer to support for "data-driven styling", not about support for "property functions" specifically. We're not going to create any properties that support property functions but not zoom-and-property functions, and most people looking for DDS support information don't care about the difference between those two types of functions.

[tmcw]

@jfirebaugh I see your objection but am not seeing a proposal to clarify the documentation.

To restate the problem without potentially incorrect wording:

• Right now we have some text that says "support for this feature varies across the API."
• This leads the user to ask "how does it vary? Where can I use the feature?"

If we say that support for "property functions" varies but we don't say "property function" anywhere in the particular property documentation, we're inviting the reader to ask a question and not providing the answer. I am amenable to any solution that fixes this problem.

[jfirebaugh]

Concretely, I suggest fixing #529 and then replacing "Note that support for property functions is not available across all properties and platforms at this time." with the following:

Support for property functions and zoom-and-property functions -- commonly known as "data-driven styling" -- varies by style property and SDK. Refer to the "data-driven styling" row in the SDK Support matrix for the property you're interested in.

[stevage]

Thanks very much for addressing this issue. I've been bitten a few times.

Things that happen to me:

• at some point in my googling, I end up on the Mapbox GL data-driven style reference but then later I'm looking in the Style Spec, and confused why I can't find the bit I remember from the style reference. Should there be some links from the style spec to the help page? Or some better organisation between them?
• other times I end up on this blog post, which maybe should have a banner on it now linking to the most up to date information
• I see this sentence under "Function": "The value for any layout or paint property may be specified as a function", and take it literally (any property), and don't see the caveats paragraphs later.
• I go looking for the "data-driven styling" section of the Style Spec and...there isn't one. There's nothing in the chapter headings on the left hand side (it's actually under "Function", but that's not obvious).

Also...tiny unrelated issue: Function "type" and Function "colorSpace" don't mention what their defaults are.

[lucaswoj]

This issue was moved to mapbox/mapbox-gl-js#4139