Improve documentation for upgrade_only

[gdevenyi]

From this section: https://github.com/zyedidia/eget#does-eget-keep-track-of-installed-binaries

if so it will only download a new version if the GitHub release is newer than the binary on your file system.

The documentation fails to explain how "newer than" is determined.

• its implied this is not by any sort of tracking of eget (no manifest)
• based on timestamp?
• based on versions?