website: Adding a note around data-sources and depends_on for 0.12 users (#26222)

* Adding not about data-sources and depends-on for 0.12 users

* Bold

* A little more markdown

* A little more markdown for data_sources in 0.12

* Some iteration based on good feedback

website/docs/configuration/data-sources.html.md

@@ -117,6 +117,9 @@ Data resources have the same dependency resolution behavior
 Setting the `depends_on` meta-argument within `data` blocks defers reading of
 the data source until after all changes to the dependencies have been applied.
 
+~> **NOTE:** **In Terraform 0.12 and earlier**, due to the data resource behavior of deferring the read until the apply phase when depending on values that are not yet known, using `depends_on` with `data` resources will force the read to always be deferred to the apply phase, and therefore a configuration that uses `depends_on` with a `data` resource can never converge. Due to this behavior, we do not recommend using `depends_on` with data resources.
+
+
 ## Multiple Resource Instances
 
 Data resources support [`count`](./resources.html#count-multiple-resource-instances-by-count)