Mark Rails/WhereExists as unsafe auto-correction

## Summary

This cop is unsafe because the behavior may change on the following case:

```ruby
Author.includes(:articles).where(articles: {id: id}).exists?
#=> Perform `eager_load` behaviour (`LEFT JOIN` query) and get result.

Author.includes(:articles).exists?(articles: {id: id})
#=> Perform `preload` behaviour and `ActiveRecord::StatementInvalid` error occurs.
```

## Additional Information

And this cop may be considered disabled by default as there is no consensus on the default style.

- rubocop#286
- rubocop#342

config/default.yml

@@ -770,6 +770,7 @@ Rails/WhereEquals:
 Rails/WhereExists:
   Description: 'Prefer `exists?(...)` over `where(...).exists?`.'
   Enabled: 'pending'
+  SafeAutoCorrect: false
   EnforcedStyle: exists
   SupportedStyles:
     - exists

docs/modules/ROOT/pages/cops_rails.adoc

@@ -4512,7 +4512,7 @@ User.where(users: { name: 'Gabe' })
 
 | Pending
 | Yes
-| Yes
+| Yes (Unsafe)
 | 2.7
 | 2.8
 |===
@@ -4525,6 +4525,17 @@ then the cop enforces `exists?(...)` over `where(...).exists?`.
 When EnforcedStyle is 'where' then the cop enforces
 `where(...).exists?` over `exists?(...)`.
 
+This cop is unsafe auto-correction because the behavior may change on the following case:
+
+[source,ruby]
+----
+Author.includes(:articles).where(articles: {id: id}).exists?
+#=> Perform `eager_load` behaviour (`LEFT JOIN` query) and get result.
+
+Author.includes(:articles).exists?(articles: {id: id})
+#=> Perform `preload` behaviour and `ActiveRecord::StatementInvalid` error occurs.
+----
+
 === Examples
 
 ==== EnforcedStyle: exists (default)

lib/rubocop/cop/rails/where_exists.rb

@@ -11,6 +11,17 @@ module Rails
       # When EnforcedStyle is 'where' then the cop enforces
       # `where(...).exists?` over `exists?(...)`.
       #
+      # This cop is unsafe auto-correction because the behavior may change on the following case:
+      #
+      # [source,ruby]
+      # ----
+      # Author.includes(:articles).where(articles: {id: id}).exists?
+      # #=> Perform `eager_load` behaviour (`LEFT JOIN` query) and get result.
+      #
+      # Author.includes(:articles).exists?(articles: {id: id})
+      # #=> Perform `preload` behaviour and `ActiveRecord::StatementInvalid` error occurs.
+      # ----
+      #
       # @example EnforcedStyle: exists (default)
       #   # bad
       #   User.where(name: 'john').exists?