Explaining what datastore funcs. do when a variable does not exist

Hello Scott,

Here's some suggested additions to '3.11.1. Functions for Accessing Datastore
Variables' to explain what happens when the variable does not exist:

  * Add this to the d.getVar() description:

      Returns None if the variable "X" does not exist.

  * Add this to the d.appendVar() description:

      Acts like d.setVar("X", "value") if the variable "X" does not exist.

  * Add this to the d.prependVar() description (same as above):

      Acts like d.setVar("X", "value") if the variable "X" does not exist.

  * Add this to the d.delVar() description:

      Does nothing if the variable "X" does not exist.

  * Add this to the d.renameVar() description:

      Does nothing if the variable "X" does not exist.

  * Add this to the d.getVarFlag() description:

      Returns None if either the variable "X" or the named flag does not exist.

  * Add this to the d.appendVarFlag() description:

      Acts like d.setVarFlag("X", flag, "value") if the named flag does not
      exist.

  * Add this to the d.prependVarFlag() description (same as above):

      Acts like d.setVarFlag("X", flag, "value") if the named flag does not
      exist.

  * Add this to the d.getVarFlags() description:

      Returns None if the variable "X" does not exist.

  * Add this to the d.delVarFlags() description:

      Does nothing if the variable "X" does not exist.

  * Add this to the d.expand() description:

      References to variables that do not exist are left as is. For example,
      d.expand("foo ${X}") expands to the literal string "foo ${X}" if the
      variable "X" does not exist.


Cheers,
Ulf