Instruction Text
Instruction text appears in the UI to tell the user what they can do and how to do it.
It can appear around buttons or other UI elements to provide context or explanations:

Tell the User What They Need to Succeed
Always tell the user of any non-"edge case" limitations in the UI. Leave unusual or edge-case situations in the documentation.
Explain foundational concepts or ideas that are necessary for where the user is in the UI.
Instruct them on how-to, or what a next step is that might be hard to find.
Direct them to required inputs or other critical information.
Keep It Short
Try and keep instruction text to under a sentence, where possible.
Link to the documentation when needed.
Do Not Write Out Numbers
Unlike technical documentation, UI copy should always use numerals for ease of recognition. Use 3 over three.