Document additional File variables

[jmooring]

The following are not listed on https://gohugo.io/variables/files/:

.File.Filename
.File.FileInfo
.File.IsZero
.File.Section

Ref: https://github.com/gohugoio/hugo/blob/master/source/fileInfo.go

[bep]

We need to hold off on this change. Most/all of this is considered internal API and not documented by purpose (especially the FileInfo.Meta struct; it is used internally in Hugo and will certainly change without notice.)