Clarify luminance gain prose (#1117).

[skynavga]

Closes #1117.

[css-meeting-bot]

The Timed Text Working Group just discussed Clarify luminance gain prose (#1117). #1156, and agreed to the following:

• SUMMARY: Glenn to update the PR to remove the extraneous units and Pierre to approve, and Glenn to merge

The full IRC log of that discussion

<cyril> Topic: Clarify luminance gain prose (#1117). #1156
<cyril> github: https://github.com/w3c/ttml2/pull/1156
<cyril> nigel: I had a philosphical debate about "determine"
<cyril> pal: I prefer "determine"
<cyril> glenn: I can take out the cd/m2 unit
<cyril> glenn: the other comment I'm not sure it's good
<cyril> pal: fine, you can ignore it, it is not that important
<cyril> SUMMARY: Glenn to update the PR to remove the extraneous units and Pierre to approve, and Glenn to merge

[skynavga]

@palemieux pls resolve conversations and approve