Add documentation note about common pitfall with the annotated pattern

Backport of: #11811

Author: Viicos

docs/concepts/fields.md

@@ -76,6 +76,24 @@ assignment form instead.
         # Invalid: [-1, 2]
     ```
 
+    Be careful not mixing *field* and *type* metadata:
+
+    ```python {test="skip" lint="skip"}
+    class Model(BaseModel):
+        field_bad: Annotated[int, Field(deprecated=True)] | None = None  # (1)!
+        field_ok: Annotated[int | None, Field(deprecated=True)] = None  # (2)!
+    ```
+
+      1. The [`Field()`][pydantic.fields.Field] function is applied to `int` type, hence the
+         `deprecated` flag won't have any effect. While this may be confusing given that the name of
+         the [`Field()`][pydantic.fields.Field] function would imply it should apply to the field,
+         the API was designed when this function was the only way to provide metadata. You can
+         alternatively make use of the [`annotated_types`](https://github.com/annotated-types/annotated-types)
+         library which is now supported by Pydantic.
+
+      2. The [`Field()`][pydantic.fields.Field] function is applied to the "top-level" union type,
+         hence the `deprecated` flag will be applied to the field.
+
 ## Default values
 
 Default values for fields can be provided using the normal assignment syntax or by providing a value