Proto extensions spec (#7584)

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## Summary of the Pull Request
Proto-extensions spec

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## PR Checklist
* [x] Is documentation
* [x] I work here
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

.github/actions/spell-check/dictionary/apis.txt

@@ -35,6 +35,7 @@ LCID
 llabs
 lround
 LSHIFT
+msappx
 NCHITTEST
 NCLBUTTONDBLCLK
 NCRBUTTONDBLCLK

.github/actions/spell-check/dictionary/dictionary.txt

@@ -313157,6 +313157,7 @@ post-Mishnical
 postmistress
 postmistresses
 postmistress-ship
+postmodern
 postmortal
 post-mortem
 postmortem