Call out gitlab:uploads:migrate:all as the most common use case

doc/administration/raketasks/uploads/migrate.md

 # Uploads migrate Rake tasks **(CORE ONLY)**
 
-`gitlab:uploads:migrate` migrates uploads between different storage types.
+There is a Rake task for migrating uploads between different storage types.
+
+- Migrate all uploads with [`gitlab:uploads:migrate:all`](#all-in-one-rake-task) or
+- To only migrate specific upload types, use [`gitlab:uploads:migrate`](#individual-rake-tasks).
 
 ## Migrate to object storage
 
@@ -166,3 +169,17 @@ To migrate uploads from object storage to local storage:
 
 After running the Rake task, you can disable object storage by undoing the changes described
 in the instructions to [configure object storage](../../uploads.md#using-object-storage-core-only).
+
+## Troubleshooting
+
+### undefined method constantize
+
+You will see the following error if you run `gitlab:uploads:migrate` without parameters.
+
+```plaintext
+rake aborted!
+NoMethodError: undefined method `constantize' for nil:NilClass
+```
+
+- If you intend to migrate all uploads, use the all-in-one Rake task [`gitlab:uploads:migrate:all`](#all-in-one-rake-task).
+- To migrate specific uploads, use [`gitlab:uploads:migrate`](#individual-rake-tasks) and supply the necessary parameters.

doc/administration/uploads.md

-# Uploads administration
+# Uploads administration **(CORE ONLY)**
 
 Uploads represent all user data that may be sent to GitLab as a single file. As an example, avatars and notes' attachments are uploads. Uploads are integral to GitLab functionality, and therefore cannot be disabled.
 
@@ -108,7 +108,7 @@ _The uploads are stored by default in
    ```
 
 1. Save the file and [reconfigure GitLab](restart_gitlab.md#omnibus-gitlab-reconfigure) for the changes to take effect.
-1. Migrate any existing local uploads to the object storage using [`gitlab:uploads:migrate` Rake task](raketasks/uploads/migrate.md).
+1. Migrate any existing local uploads to the object storage using [`gitlab:uploads:migrate:all` Rake task](raketasks/uploads/migrate.md).
 
 **In installations from source:**
 
@@ -131,7 +131,7 @@ _The uploads are stored by default in
    ```
 
 1. Save the file and [restart GitLab](restart_gitlab.md#installations-from-source) for the changes to take effect.
-1. Migrate any existing local uploads to the object storage using [`gitlab:uploads:migrate` Rake task](raketasks/uploads/migrate.md).
+1. Migrate any existing local uploads to the object storage using [`gitlab:uploads:migrate:all` Rake task](raketasks/uploads/migrate.md).
 
 ### OpenStack example
 
@@ -157,7 +157,7 @@ _The uploads are stored by default in
    ```
 
 1. Save the file and [reconfigure GitLab](restart_gitlab.md#omnibus-gitlab-reconfigure) for the changes to take effect.
-1. Migrate any existing local uploads to the object storage using [`gitlab:uploads:migrate` Rake task](raketasks/uploads/migrate.md).
+1. Migrate any existing local uploads to the object storage using [`gitlab:uploads:migrate:all` Rake task](raketasks/uploads/migrate.md).
 
 ---
 
@@ -188,4 +188,4 @@ _The uploads are stored by default in
    ```
 
 1. Save the file and [reconfigure GitLab](restart_gitlab.md#omnibus-gitlab-reconfigure) for the changes to take effect.
-1. Migrate any existing local uploads to the object storage using [`gitlab:uploads:migrate` Rake task](raketasks/uploads/migrate.md).
+1. Migrate any existing local uploads to the object storage using [`gitlab:uploads:migrate:all` Rake task](raketasks/uploads/migrate.md).