DOCSP-35833: New LINQ methods (mongodb#364)

* DOCSP-35833: New LINQ methods

* MR feedback

(cherry picked from commit 7f3efc0)

Author: mcmorisi

source/fundamentals/linq.txt

@@ -805,6 +805,9 @@ The following are some methods supported by the {+driver-long+} implementation o
 
    * - ``Distinct``
      - Returns distinct documents that match the specified criteria
+   
+   * - ``Exists``
+     - Tests whether a field exists
 
    * - ``DistinctMany``
      - Returns distinct documents from an array that match the specified criteria
@@ -824,6 +827,12 @@ The following are some methods supported by the {+driver-long+} implementation o
    * - ``GroupJoin``
      - Performs a left outer join to another collection in the same database
 
+   * - ``IsMissing``
+     - Returns ``true`` if a field is missing and false otherwies
+   
+   * - ``IsNullOrMissing``
+     - Returns ``true`` if a field is null or missing and false otherwise
+
    * - ``Max``
      - Returns the document with the maximum specified value