RUBY-2861 add documentation for the loss of trailing zeroes (#274)

* RUBY-2861 add docs for the loss of trailing zeroes

* RUBY-2861 add note about future on Decimal128

* RUBY-2861 update language

* RUBY-2861 update language

Author: neilshweky

docs/tutorials/bson-v4.txt

@@ -543,6 +543,17 @@ follows:
   ``BigDecimal`` is not. All signed ``NaN`` values that are deserialized into
   ``BigDecimal`` instances will be unsigned.
 
+- ``BSON::Decimal128`` maintains trailing zeroes when serializing to and
+  deserializing from BSON. ``BigDecimal``, however, does not maintain trailing
+  zeroes and therefore using ``BigDecimal`` may result in a lack of precision.
+
+.. note::
+
+  In BSON 5.0, ``BSON::Decimal128`` will be deserialized into ``BigDecimal`` by
+  default. In order to have ``BSON::Decimal128`` values in BSON documents
+  deserialized into ``BSON::Decimal128``, the ``mode: :bson`` option can be set
+  on ``from_bson``.
+
 ``Symbol``
 ----------