fix(@angular/cli): adding help descriptions

[sumitarora]

Shuld Fix: #4845

[filipesilva]

Good job getting these all in!

In the future we should make the command descriptions the source of truth for the wiki docs even, similar to how the completion command uses options to auto-generate shell scripts.

I've a couple of change requests:

• Use oneLine (or similar, as needed) from the common-tags lib for multiline descriptions instead of concatenating strings.
• Start every description with a capital letter, and end it with a period.
• Remove flag to... (and similar) from descriptions. e.g. flag whether to build using Ahead of Time compilation -> build using Ahead of Time compilation. I know that's how it shows up in our doc but its pretty redundant.
• If you have time, propagate your new description strings back to the wiki doc.

[sumitarora]

@filipesilva Done the changes to make changes back to wiki will talk to @Brocco

[filipesilva]

Awesome, thanks!

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