RUBY-2880 Implement driver-compatible DBRef constructor (#282)

Co-authored-by: Oleg Pudeyev <[email protected]>

Author: p-mongo

docs/tutorials/bson-v4.txt

@@ -405,8 +405,9 @@ Represents a string of JavaScript code with a hash of values.
 
   BSON::CodeWithScope.new("this.value = age;", age: 5)
 
+
 ``BSON::DBRef``
-------------------
+---------------
 
 This is a subclass of ``BSON::Document`` that provides accessors for the
 collection, id, and database of the DBRef.
@@ -437,6 +438,16 @@ collection, id, and database of the DBRef.
     loaded.class
     => BSON::DBRef
 
+For backwards compatibility with the MongoDB Ruby driver versions 2.17 and
+earlier, ``BSON::DBRef`` also can be constructed using the legacy driver API.
+This API is deprecated and will be removed in a future version of ``bson-ruby``:
+
+.. code-block:: ruby
+
+  BSON::DBRef.new("collection", BSON::ObjectId('61eeb760a15d5d0f9f1e401d'))
+  BSON::DBRef.new("collection", BSON::ObjectId('61eeb760a15d5d0f9f1e401d'), "db")
+
+
 ``BSON::Document``
 ------------------