Show default when flags toggle feature

**What is wrong with the docs?**
The `cabal build --help` and other help output shows flags for enabling and disabling features but doesn't say what the default is. Could we indicate which is the default?

```diff
$ cabal build --help
...
--enable-tests Enable dependency checking and compilation for
test suites listed in the package description
file.
- --disable-tests Disable dependency checking and compilation
+ --disable-tests (default) Disable dependency checking and compilation
for test suites listed in the package
description file.
```

Start by running `cabal build --help` and compare the paired enable/disable feature flags, including `--enable-tests` and `--disable-tests`. Trace where this help output is generated, then update the relevant help text so the default choice is marked consistently and verify the resulting command output.

Written by the indexing model from the issue text.