Tracking: document `--prod` and add minification flag

[filipesilva]

We should document what --prod really does.

It's actually a sort of meta-flag that adds a bunch of defaults: https://github.com/angular/angular-cli/blob/master/packages/@angular/cli/models/webpack-config.ts#L84-L89

Besides setting these defaults, it also adds minification and a few more things: https://github.com/angular/angular-cli/blob/master/packages/@angular/cli/models/webpack-configs/production.ts#L85-L93

Minification should perhaps be opt-in however, via a flag.

[johnnliu]

I was able to figure out that --prod seem to have done AOT and -environment=prod but wasn't sure what else it did.

The really shocking part was actually that I find the documentation missing and that filled me with worry.

Thanks for clarifications on twitter.

[sumitarora]

@filipesilva we can add that in build command wiki ?

[filipesilva]

@sumitarora I'm adding the docs bit in #6232, can you add the minification flag while you're fixing the UglifyJS problems you are working on?