Show default when flags toggle feature

[philderbeast]

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?

   $ 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.

[Bodigrim]

I think we should aim for #5607 first, before putting more tweaks into command-line parsing.

[philderbeast]

Command line flags come after all other settings and override them. If we do mark a flag as default this would be assuming no other settings are in effect. These annotations show a situation like this;

   $ cabal build --help
   ...
-   --enable-tests                 Enable dependency checking and compilation for
+   --enable-tests (already enabled by project setting in cabal.project)       
+                                  Enable dependency checking and compilation for
                                   test suites listed in the package description
                                   file.
-   --disable-tests                Disable dependency checking and compilation
+   --disable-tests (override effective setting)
+                                  Disable dependency checking and compilation
                                   for test suites listed in the package
                                   description file.

[Mikolaj]

Let's discuss which is better, the "default" hint or the more detailed (and, arguably, more honest) but longer and more cryptic notes.