Present tense

In general, use present tense rather than future tense; in particular, try to avoid using will where possible. For example:

Not recommended: Send a query to the service. The server will send an acknowledgment.

Recommended: Send a query to the service. The server sends 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.

But 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 "Recommended" example above.

Also avoid the hypothetical future would. For example:

Not recommended: You can send an unsubscribe message. The server would then remove you from the mailing list.

Recommended: If you send an unsubscribe message, the server removes you from the mailing list.