MONGOID-5381 Correct map_big_decimal_to_decimal128 feature flag default and write 8.0 release note (#5338)

* MONGOID-5381 add BigDecimal release notes

* MONGOID-5381 change the default for feature flag

* MONGOID-5381 remove unused code

* MONGOID-5381 fix failing test

* MONGOID-5381 flip flag

* MONGOID-5381 adjust release note

Author: neilshweky

source/reference/configuration.txt

@@ -397,8 +397,8 @@ for details on driver options.
       log_level: :info
 
       # When using the BigDecimal field type, store the value in the database
-      # as a BSON::Decimal128 instead of a string. (default: false)
-      map_big_decimal_to_decimal128: false
+      # as a BSON::Decimal128 instead of a string. (default: true)
+      #map_big_decimal_to_decimal128: true
 
       # Force ``BSON::ObjectId#as_json`` method to return the hash
       # { "$oid" => id.to_s }. When this option is false, and bson-ruby 5

source/reference/fields.txt

@@ -428,6 +428,8 @@ matches strings containing "hello" before a newline, besides strings ending in
 This is because the meaning of ``$`` is different between PCRE and Ruby
 regular expressions.
 
+.. _bigdecimal-fields:
+
 BigDecimal Fields
 -----------------
 

source/release-notes/mongoid-8.0.txt

@@ -40,7 +40,6 @@ changed in Mongoid 8.0:
 - ``:legacy_attributes`` => true
 - ``:legacy_pluck_distinct`` => ``false``
 - ``:legacy_triple_equals`` => ``false``
-- ``:map_big_decimal_to_decimal128`` => ``true``
 - ``:object_id_as_json_oid`` => ``false``
 - ``:overwrite_chained_operators`` => ``false``
 
@@ -472,3 +471,17 @@ it was stored and returned as a String.
   p registry.data
   # => Mongoid 7: "data!"
   # => Mongoid 8: <BSON::Binary:0x2600 type=generic data=0x6461746121...>
+
+``Decimal128``-backed ``BigDecimal`` fields
+-------------------------------------------
+
+Mongoid 8 introduces the ``map_big_decimal_to_decimal128`` feature flag, which
+allows values assigned to a field of type ``BigDecimal`` to be stored as type
+``String`` in the database for compatibility with Mongoid 7 and earlier. In
+Mongoid 8 by default (with this feature flag turned on), values assigned to
+fields of type ``BigDecimal`` are stored in the database as type
+``BSON::Decimal128``. In Mongoid 7 and earlier, and in Mongoid 8 with this
+feature flag turned off, values assigned to fields of type ``BigDecimal`` are
+stored as Strings. See the section on :ref:`BigDecimal Fields <bigdecimal-fields>`
+for more details.
+