message: Clarify ResourceVariable specification
diff: @@ -171,7 +171,9 @@ class ResourceVariable(variables.Variable):
  to see all modifications to the value of the variable which happen in any
  operation on which the read_value depends on (either directly, indirectly, or
  via a control dependency) and guaranteed to not see any modification to the
  - value of the variable on which the read_value operation does not depend on.
  + value of the variable from operations that depend on the read_value operation.
  + Updates from operations that have no dependency relationship to the read_value
  + operation might or might not be visible to read_value.
  For example, if there is more than one assignment to a ResourceVariable in
  a single session.run call there is a well-defined value for each operation
  