Keep It Clean
Anyone who has been using Ant for any reasonable length of time will understand what targets, filesets, and tasks are used for. Therefore, it is unnecessary to have a large number of comments in the build file that details every single action. Avoid this by having clear and concise comments in the build file.
If the target's purpose is not clear (such as clean or compile), explain what the target does briefly. Don't go overboard with comments that will end up just cluttering the file and making it hard to read. Also, an overabundance of comments tends to become outdated as the project continues (because comments are usually not updated when the targets are changed).