Docsp 21619 v5.3 backport (#1403)

* DOCSP-21619 clarification on getUser

* DOCSP-21619 updating tense and phrasing

* DOCSP-21619 fixing method format

Author: ltran-mdb

source/reference/method/db.getUser.txt

@@ -16,8 +16,8 @@ Definition
 .. method:: db.getUser(username, args)
 
    Returns user information for a specified user. Run this method on the
-   user's database. The user must exist on the database on which the method
-   runs.
+   user's database. If the user doesn't exist in the database,
+   :method:`db.getUser()` returns ``null``. 
 
    The :method:`db.getUser()` method has the following parameters:
 
@@ -136,6 +136,7 @@ Example output:
       mechanisms: [ 'SCRAM-SHA-1', 'SCRAM-SHA-256' ]
    }
 
+
 Omit Custom Data from Output
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~