[Slim] Proposed 5.4 text

[Randall Gellens <[e-mail]>]

Based on the text and comments from Gunnar, brian, Paul, and Bernard, 
here is what I propose to replace section 5.4 with:

5.4.  Usage Notes

    A sign language tag with a video stream is interpreted as an
    indication for sign language in the video stream.  A non-sign
    language tag with text media is interpreted as an indication for
    written language.  A non-sign language tag with audio media is
    interpreted as an indication for spoken language.

    In the IANA registry of language subtags per BCP 47 [RFC5646], a
    language subtag with a Type field "extlang" combined with a Prefix
    field value "sgn" indicates a sign language tag.  The absense of
    such "sgn" prefix indicates a non-sign language tag.  This document
    does not define any other use for language tags in video media
    (such as how to indicate visible captions).

    This document does not define the use of sign language tags in text
    or audio media.

    This document does not define the use of language tags in media
    other than interactive streams of audio, video, and text (such as
    "message" or "application").

-- 
Randall Gellens
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