[TextField] Fix a warning of 'Failed prop type'

[szchenghuang]

TextField passes prop hintText to EnhancedTextArea. The former defines its type as PropTypes.node while the later expects PropTypes.string, resulting in the warning.

Fix #6747.

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

[oliviertassinari]

@szchenghuang Thanks