Full definition of .cabal file syntax

Is this what you want?

Distribution.PackageDiscription

a proper grammar definition for the .cabal file format.

The grammar is defined by its parser. I don't know of a formal specification.

The list of the fields, whether they are required or not, and what goes in them is here https://www.haskell.org/cabal/users-guide/developing-packages.html#package-properties.

Not the most obvious of places, I must admit.