Add usage notes about + prefix

website/docs/reference/advanced-config-usage.md

@@ -40,3 +40,53 @@ select ...
 ## Hierarchies / overriding configs / precedence
 For Drew to do
 --->
+
+## Using the `+` prefix
+
+Throughout this documentation, we've tried to be consistent in using the `+` prefix to differentiate between a configuration, and part of a [resource path](resource-path) in `dbt_project.yml` files. This was introduced in dbt v0.17.0. For example:
+
+<File name='dbt_project.yml'>
+
+```yml
+
+models:
+  +materialized: view
+  jaffle_shop:
+    marts:
+      +materialized: table
+```
+
+</File>
+
+The leading `+` is in fact _only required_ when you need to disambiguate between resource paths and configs, for example when:
+- A config accepts a dictionary as its inputs, for example, the [`persist_docs` config](persist_docs).
+- Or, a config shares a key with part of a resource path, for example, if you had a directory of models named `tags`.
+
+<File name='dbt_project.yml'>
+
+```yml
+name: jaffle_shop
+config-version: 2
+
+...
+
+models:
+  +persist_docs: # this config is a dictionary, so needs a + prefix
+    relation: true
+    columns: true
+
+  jaffle_shop:
+    +tags: # this is the tag config
+      - "hello"
+    tags: # whereas this is the tag resource path
+      # The below config applies to models in the
+      # models/tags/ directory.
+      # Note: you don't _need_ a leading + here,
+      # but it wouldn't hurt.
+      materialized: view
+
+```
+
+</File>
+
+However it doesn't hurt to use the `+` prefix, so we recommend you use it whenever adding configs to your `dbt_project.yml` file.