ixgbe.4: Add flow director CAVEAT

Flow-director support is not fully implemented and will cause
errors if enabled. Mention this in the ixgbe(4) manual.

PR: 202663
MFC after: 3 days
Co-authored-by: Alexander Ziaee <concussious@runbox.com>
Pull Request: https://github.com/freebsd/freebsd-src/pull/1493