Errata on non-prohibition of partially supported features

[palemieux]

See #488

[palemieux]

Summary:

The following paragraph is added to Section 4:

A Feature or Extension designated as partially supported is neither completely prohibited nor
completely permitted. Instead, the disposition of each of its subset Feature or Extension is specified
individually.

[css-meeting-bot]

The Timed Text Working Group just discussed Errata on non-prohibition of partially supported features imsc#500, and agreed to the following:

• RESOLUTION: Publish this erratum as at https://github.com/w3c/imsc/issues/500#issuecomment-562895784

The full IRC log of that discussion

<nigel> Topic: Errata on non-prohibition of partially supported features imsc#500
<nigel> github: https://github.com//issues/500
<nigel> Nigel: This is one we already added to IMSC 1.2, defining partial support for a feature.
<nigel> .. Any objections to proceeding with this erratum?
<nigel> group: [no objections]
<nigel> RESOLUTION: Publish this erratum as at https://github.com//issues/500#issuecomment-562895784
<nigel> Glenn: I would not say this helps readers but it does somewhat resolve the ambiguity.