Change the types used on macOS

[avidrissman]

This specification is not the work of web.com, and thus using the com.web.* prefix is inappropriate. Change the clipboard types used to begin with org.w3 as this specification is theirs.

Also add macOS specific clarification.

Closes #412

The following tasks have been completed:

• [ ] Confirmed there are no ReSpec/BikeShed errors or warnings.
• [ ] Modified Web platform tests (link to pull request)

Implementation commitment:

• [ ] WebKit (https://bugs.webkit.org/show_bug.cgi?id=)
  • I don’t believe this spec is currently implemented in WebKit, and therefore the implementation commitment for this PR is subsumed by the implementation commitment for this spec and is inapplicable
• [X] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=1401040)
  • A CL, https://chromium-review.googlesource.com/c/chromium/src/+/4108170, is complete
• [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)
  • I don’t believe this spec is currently implemented in Gecko, and therefore the implementation commitment for this PR is subsumed by the implementation commitment for this spec and is inapplicable

[snianu]

The changes look good to me, but before we merge this change, we want to discuss with the EditingWG members to see if they have any feedback.

[avidrissman]

Certainly! Please reach out if the EditingWG folks have any questions.

[snianu]

We discussed this issue and no concerns were raised by the EditingWG members. Thank you @avidrissman for working on this!

[avidrissman]

Thank you, @snianu, for helping shepherd this along. https://crrev.com/c/4108170 is in the commit queue and should be landed soon.

Good luck in the larger task of getting this explainer merged upstream.