Make coder aware of field_filter_xss()

Coder Review yields a critical false positive if the title or description of a form API array is filtered with field_filter_xss() instead of filter_xss() / check_plain() etc.:

Potential problem: FAPI elements '#title' and '#description' only accept filtered text, be sure to use check_plain(), filter_xss() or similar to ensure your $variable is fully sanitized.

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