MONGOID-5100 allow selector arguments for .exists? (#5466)

* MONGOID-5100 allow selector arguments for .exists?

* MONGOID-5100 add examples to the docstrings

* MONGOID-5100 add docs and release notes

* Empty-Commit

* MONGOID-5100 allow false to be passed

* MONGOID-5100 update docs and docstrings

* MONGOID-5100 remove false as a param

* MONGOID-5100 put back nil/false

* MONGOID-5100 fix memory tests and fix limit == 0

Author: neilshweky

source/reference/queries.txt

@@ -1243,11 +1243,22 @@ Mongoid also has some helpful methods on criteria.
        *Determine if any matching documents exist. Will return true if there
        are 1 or more.*
 
+       ``#exists?`` *now takes a number of argument types:*
+
+       - ``Hash``: *A hash of conditions.*
+       - ``Object``: *An _id to search for.*
+       - ``false``/``nil``: *Always returns false.*
+
      -
         .. code-block:: ruby
 
           Band.exists?
           Band.where(name: "Photek").exists?
+          Band.exists?(name: "Photek")
+          Band.exists?(BSON::ObjectId('6320d96a3282a48cfce9e72c'))
+          Band.exists?('6320d96a3282a48cfce9e72c')
+          Band.exists?(false)
+          Band.exists?(nil)
 
    * - ``Criteria#fifth``
 

source/release-notes/mongoid-8.1.txt

@@ -287,3 +287,16 @@ raised when destroying or deleting, and not on saving or updating. See the
 section on :ref:`Read-only Documents <readonly-documents>` for more details.
 
 
+Added method parameters to ``#exists?``
+---------------------------------------
+
+Mongoid 8.1 introduces method paramters to the ``Contextual#exists?`` method.
+An _id, a hash of conditions, or ``false``/``nil`` can now be included:
+
+.. code:: ruby
+
+  Band.exists?
+  Band.exists?(name: "The Rolling Stones")
+  Band.exists?(BSON::ObjectId('6320d96a3282a48cfce9e72c'))
+  Band.exists?(false) # always false
+  Band.exists?(nil) # always false