docs: Minor expansion of application order docs

The discussion in #3198 suggests some confusion about the order of
application. This adds a little more context.

assets/chezmoi.io/docs/reference/application-order.md

@@ -6,8 +6,10 @@ chezmoi is deterministic in its order of application. The order is:
 2. Read the destination state.
 3. Compute the target state.
 4. Run `run_before_` scripts in alphabetical order.
-5. Update entries in the target state (files, directories, scripts, symlinks,
-   etc.) in alphabetical order of their target name.
+5. Update entries in the target state (files, directories, externals, scripts,
+   symlinks, etc.) in alphabetical order of their target name. Directories
+   (including those created by externals) are updated before the files they
+   contain.
 6. Run `run_after_` scripts in alphabetical order.
 
 Target names are considered after all attributes are stripped.
@@ -26,3 +28,9 @@ chezmoi's behavior when the above assumptions are violated is undefined. For
 example, using a `run_before_` script to update files in the source or
 destination states violates the assumption that the source and destination
 states do not change while chezmoi is running.
+
+!!! note
+
+    External sources are updated during the update phase; it is inadvisable for
+    a `run_before_` script to depend on an external applied *during* the update
+    phase. `run_after_` scripts may freely depend on externals.