The notes from the CRAN maintainer were:

> The Description field is intended to be a (one paragraph) description of what the package does and why it may be useful. Please add more details about the package functionality and implemented methods in your Description text.

The only guidelines I found otherwise were:

 * It is a single paragraph.
 * It is informative and concise, and it clearly describes what the package does and why it is useful.
 * It does not include the package name.
 * It does not start with "A package for ..." or "This package does ...".
 * It is written in plain text, with no markup.
 * It is indented correctly, with each line no more than 80 characters wide.
 * It includes relevant citations, formatted in the correct style.
 * It is free of errors in grammar and spelling.