In general, use present tense rather than future tense; in particular, try to avoid using will where possible—for example:
Recommended: Send a query to the service. The server sends an acknowledgment.
Not recommended: Send a query to the service. The server will send an acknowledgment.
Sometimes, of course, future tense is unavoidable because you're actually talking about the future (for example, The hedgehog will hibernate starting in October.). Attempting to predict the future in a document is usually a bad idea, but sometimes it's necessary.
However, the fact that the reader will be writing and running code in the future isn't a good reason to use future tense. Stick with present tense where possible, as shown in the previous Recommended example.
Also avoid the hypothetical future would—for example:
Recommended: If you send an unsubscribe message, the server removes you from the mailing list.
Not recommended: You can send an unsubscribe message. The server would then remove you from the mailing list.