Document the data source interpolation usage (#14144)

* Document the data source interpolation usage

Also briefly mentions how to use counts and splat syntax as with resources to further document the usage of counts for data sources (see https://github.com/hashicorp/terraform/pull/14143).

* Output -> Attribute

As per feedback.

website/source/docs/configuration/interpolation.html.md

@@ -61,6 +61,13 @@ attribute set, you can access individual attributes with a zero-based
 index, such as `${aws_instance.web.0.id}`. You can also use the splat
 syntax to get a list of all the attributes: `${aws_instance.web.*.id}`.
 
+#### Attributes of a data source
+
+The syntax is `data.TYPE.NAME.ATTRIBUTE`. For example. `${data.aws_ami.ubuntu.id}` will interpolate the `id` attribute from the `aws_ami` [data source](/docs/configuration/data-sources.html) named `ubuntu`. If the data source has a `count`
+attribute set, you can access individual attributes with a zero-based
+index, such as `${data.aws_subnet.example.0.cidr_block}`. You can also use the splat
+syntax to get a list of all the attributes: `${data.aws_subnet.example.*.cidr_block}`.
+
 #### Outputs from a module
 
 The syntax is `MODULE.NAME.OUTPUT`. For example `${module.foo.bar}` will