Add userName and password hint options for login

[jarrodwhitley]

Add userName and password hint options to the Dialogs.login example.

PR Checklist

• [x] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
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What is the current state of the documentation article?

The current doc does not mention or demonstrate the available input placeholders via the userNameHint and passwordHint options.

What is the new state of the documentation article?

I added both options to the existing example

Fixes/Implements/Closes #[Issue Number].

n/a

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[jarrodwhitley]

@cla-bot check

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