what should be in the git description file?

description file is only used by the GitWeb program ( to display the description of the repo on the GitWeb page)

https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Porcelain

Otherwise, how you use it is left to you and I don't see a point in a "style" for it.

The description file can also be read by hook scripts or other scripts pertaining to the repository. For example, one popular hook script, post-receive-email, sends an email to all members when a commit is made to the repository, and uses the contents of the description file as the name of the repository in the email subject.

description is just a text file that is shown as project description in web frontend.

Just write there something that explains what the repository is about.

The content of this file is also used by gitolite with the "desc -ld" command.

you can modify this file, but it doesn't seem to me that it's taken into account to be commited.