`cabal help user-config` should mention both config paths

**What is wrong with the docs?**
A user on Libera `#haskell` noted that `cabal help user-config` talks about `~/.config/cabal/config` even when the user running it has a legacy `~/.cabal/config` instead.

Start by locating the help text produced by `cabal help user-config` and inspect how it documents the configuration path. Update the wording to mention both `~/.config/cabal/config` and the legacy `~/.cabal/config`, then run the relevant help or CLI tests to confirm both paths appear.

Written by the indexing model from the issue text.