Improve use_big_decimal_serializer documentation

Follow-up to rails#45618.

This fixes a few typos and clarifies the wording.

Author: jonathanhefner

guides/source/configuring.md

@@ -2258,14 +2258,21 @@ an `around_perform`. The default value is `true`.
 
 #### `config.active_job.use_big_decimal_serializer`
 
-Enable the use of BigDecimalSerializer instead of legacy BigDecimal argument
-serialization, which may result in the argument being lossfully converted to
-a String when using certain queue adapters.
-This setting is disabled by default to allow race condition free deployment
-of applications with multiple replicas, in which an old replica would not
-support BigDecimalSerializer..
-In such environments, it should be safe to enable this setting following
-successful deployment of Rails 7.1 which introduces BigDecimalSerializer.
+Enables the new `BigDecimal` argument serializer, which guarantees
+roundtripping. Without this serializer, some queue adapters may serialize
+`BigDecimal` arguments as simple (non-roundtrippable) strings.
+
+WARNING: When deploying an application with multiple replicas, old (pre-Rails
+7.1) replicas will not be able to deserialize `BigDecimal` arguments from this
+serializer. Therefore, this setting should only be enabled after all replicas
+have been successfully upgraded to Rails 7.1.
+
+The default value depends on the `config.load_defaults` target version:
+
+| Starting with version | The default value is |
+| --------------------- | -------------------- |
+| (original)            | `false`              |
+| 7.1                   | `true`               |
 
 ### Configuring Action Cable
 

railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_7_1.rb.tt

@@ -48,12 +48,12 @@
 # Disable deprecated singular associations names
 # Rails.application.config.active_record.allow_deprecated_singular_associations_name = false
 
-# Enable the use of BigDecimalSerializer instead of legacy BigDecimal argument
-# serialization, which may result in the argument being lossfully converted to
-# a String when using certain queue adapters.
-# This setting is disabled by default to allow race condition free deployment
-# of applications with multiple replicas, in which an old replica would not
-# support BigDecimalSerializer..
-# In such environments, it should be safe to enable this setting following
-# successful deployment of Rails 7.1 which introduces BigDecimalSerializer.
+# Enable the Active Job `BigDecimal` argument serializer, which guarantees
+# roundtripping. Without this serializer, some queue adapters may serialize
+# `BigDecimal` arguments as simple (non-roundtrippable) strings.
+#
+# When deploying an application with multiple replicas, old (pre-Rails 7.1)
+# replicas will not be able to deserialize `BigDecimal` arguments from this
+# serializer. Therefore, this setting should only be enabled after all replicas
+# have been successfully upgraded to Rails 7.1.
 # Rails.application.config.active_job.use_big_decimal_serializer = true