Document the version constraint syntax

I was looking for a way to exclude a certain version, and was wondering if there was something like a negation operator. I couldn't find a description of the constraint syntax though.

Start by locating the documentation that describes Cabal version constraints and reviewing the existing constraint syntax coverage. Determine where exclusion or negation belongs in that documentation, then add a clear syntax explanation and examples so users can understand how to express the requested constraint.

Written by the indexing model from the issue text.