DOCSP-42139: Update read concern values (#69)

(cherry picked from commit 2d45935)

Author: norareidy

source/reference/create-client.txt

@@ -662,8 +662,8 @@ Ruby Options
 
    * - ``:read_concern``
      - Specifies the read concern options. The only valid key is ``level``,
-       for which the valid values are ``:local``, ``:majority``, and
-       ``:snapshot``.
+       for which the valid values are ``:local``, ``:available``, ``:majority``,
+       ``:snapshot``, and ``:linearizable``.
      - ``Hash``
      - none