Add informative note to tts:fontStyle recommending use of tts:shear instead of tts:fontStyle="italic" for Japanese text.

[himorin]

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§ w3c/imsc#495

to add a note to recommend that the content author specify tts:shear on Japanese text instead of tts:fontStyle="italic". (following note in CSS font-style for italic fallback

[xfq]

Non-normative added in PR w3c/imsc#514

New spec text: https://w3c.github.io/imsc/imsc1/spec/ttml-ww-profiles.html#text-italic-shear-oblique