docs: Clarify docstring Warnings section usage

Author: AntoineD

docs/reference/docstrings.md

@@ -468,6 +468,14 @@ def foo():
 TIP: **Warnings names are resolved using the function's scope.**
 `UserWarning` and other built-in warnings are resolved as such. You can document custom warnings, using the names available in the current scope, for example `my_warnings.MyCustomWarning` or `MyCustomWarning` directly, depending on what you imported/defined in the current module.
 
+TIP: **Warnings section are not Warning admonitions!**
+To create a warning [admonition/callout][google-admonitions], use the singular form:
+
+```
+Warning:
+    This is a warning.
+```
+
 #### Yields {#google-section-yields}
 
 - Multiple items allowed