Add explanatory comments to cabal.project file

Author: mpilgrem

cabal.project

@@ -1,3 +1,18 @@
--- With cabal projects, import is available with cabal-install >= 3.8.
+-- This file is a configuration file for Cabal (the tool). It is provided to
+-- assist some users of that tool to develop Stack. For information about
+-- `cabal.project` files, see:
+-- https://cabal.readthedocs.io/en/stable/cabal-project.html.
+
+-- For information about possible limitations of the `cabal.config` files
+-- corresponding to Stackage package sets that are made available by Stackage
+-- see: https://github.com/fpco/stackage-server/issues/232.
+
+-- `import:` is only available to users of Cabal (the tool) >= 3.8.1.0.
+
 import: https://www.stackage.org/nightly-2022-09-05/cabal.config
-packages: .
+
+-- Extra-deps (if any) would be specified by adding:
+--
+-- constraints: <package_name> == <version_number>
+
+packages: .