Add poll documentation

[filipesilva]

The CLI has an undocumented poll option that helps in some environments, such as Vagrant. We should document it.

PRs for reference: #1814, #1837, #2486.

[sumitarora]

Closing has been added to the documentation.

[proverbed]

Good day, where is this documentation?

[proverbed]

Got it: defaults.poll

https://github.com/angular/angular-cli/pull/1814/files

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