{foo,bar} /= {foo, bar}, parse of field 'packages' failed

https://github.com/haskell/cabal/blob/3752c3e5edc3c19b7262cdb0b70f4e0539b46362/Cabal/doc/cabal-project.rst#L52-L59

The example given in the docs works. If instead I try `../{foo, bar}/` with a comma and space, this fails with:

```
cabal: Error parsing project file
/.../cabal.project:3:
Parse of field 'packages' failed.
```

I get the same error with `../{foo bar}/`.

Could we have a better error message here about why the parsing failed or a note in the docs about not using spaces within the curlies please? This is an easy trap to fall into as the section in the docs mentions "package location list (space or comma separated)".

https://github.com/haskell/cabal/blob/3752c3e5edc3c19b7262cdb0b70f4e0539b46362/Cabal/doc/cabal-project.rst#L40-L41

I scanned this repo for `cabal.project` files and could not find a test that exercises the `{foo,bar}` syntax.