There are clear iOS guidelines for creating accessibility labels. For example, the label "should begin with a capital word and not end with a period."
UITextField text input control includes the option for placeholder text that is displayed faintly when the field is empty. I haven't found guidelines for this placeholder text except for:
Display a hint in the text field if it helps users understand its purpose. A text field can display placeholder text—such as Name (or Address)—when there is no other text in the field.
As a rule of thumb, does it makes sense to apply the guidelines for accessibility labels to placeholder text also? Are there guidelines for placeholder text?