message: Add migration doc for variable initializers
diff: @@ -3263,6 +3263,11 @@ def variables_initializer(var_list, name="init"):
  If `var_list` is empty, however, the function still returns an Op that can
  be run. That Op just has no effect.
  + @compatibility(TF2)
  + In TF2, variables are initialized immediately when they are created. There is
  + no longer a need to run variable initializers before using them.
  + @end_compatibility
  +
  Args:
  var_list: List of `Variable` objects to initialize.
  name: Optional name for the returned operation.
  @@ -3289,6 +3294,11 @@ def global_variables_initializer():
  This is just a shortcut for `variables_initializer(global_variables())`
  + @compatibility(TF2)
  + In TF2, variables are initialized immediately when they are created. There is
  + no longer a need to run variable initializers before using them.
  + @end_compatibility
  +
  Returns:
  An Op that initializes global variables in the graph.
  """
  @@ -3311,6 +3321,11 @@ def local_variables_initializer():
  This is just a shortcut for `variables_initializer(local_variables())`
  + @compatibility(TF2)
  + In TF2, variables are initialized immediately when they are created. There is
  + no longer a need to run variable initializers before using them.
  + @end_compatibility
  +
  Returns:
  An Op that initializes all local variables in the graph.
  """
  