DOCS-14222 deprecates getnonce command

Author: jmd-mongo

source/reference/command/getnonce.txt

@@ -12,7 +12,13 @@ getnonce
 
 .. dbcommand:: getnonce
 
-   Client libraries use :dbcommand:`getnonce`  to generate a one-time
+   .. deprecated:: 4.0 
+
+   Client libraries use :dbcommand:`getnonce` to generate a one-time
    password for authentication.
+   
+   Applications should not invoke :dbcommand:`getnonce` directly.
+
+   This command will be removed in a future release.   
 
    .. slave-ok