Warn about exposing the actual password stings in the password component documentation

[Legioth]

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The password field documentation for Vaadin 8 warns about the fact that passwordField.setValue(secretPassword) will cause the password to be sent to the client and that the masking in the field is just a visual feature. We should have something similar also in the documentation for newer versions of the component.

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[Legioth]

See vaadin/framework#10544 for a discussion about the feature in Vaadin 8.