[docs][fidl] Add reference to API doc doc in FIDL rubric.

Test: looked at gitiles.
Change-Id: Ia68cfb8aeed41fb332d13115e9e1b91a095f282d
diff --git a/docs/development/api/fidl.md b/docs/development/api/fidl.md
index 4d99677..30ecc3f 100644
--- a/docs/development/api/fidl.md
+++ b/docs/development/api/fidl.md
@@ -282,6 +282,9 @@
 [Doc = "and so is this"]
 ```
 
+For more information about what your comments should contain, consult the [API
+Documentation Rubric](documentation.md).
+
 #### Flow-through vs. regular comment guidelines
 
 For flow through comments, the `///` form is preferred over the `[Doc = ]`