Updated the platform channels documentation for background isolate channels

[gaaclarke]

This adds documentation for the new feature that allows us to use plugins and channels from background isolates.

issue: flutter/flutter#13937

Do not land until that feature is on stable.

Presubmit checklist

• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[sfshaza2]

lgtm once some minor suggestions are addressed

[Leedehai]

Hi - any movements on this PR?

[stuartmorgan]

@Leedehai No, per the comment in the PR description:

Do not land until that feature is on stable.

[atsansone]

@gaaclarke : Did this hit stable yet?

[gaaclarke]

@gaaclarke : Did this hit stable yet?

no, siree

[sfshaza2]

@gaaclarke, I see that issue flutter/flutter#13937 is closed because it has been fixed on master. Do you know when it will hit stable? I see a few possible approaches:

• We add a note to the text that this is fixed on master and then remove when it hits stable
• We add a note that it will be stable in release 3.3.8 (or whatever)
• We leave this PR as is until it does actually hit stable

I'm not a fan of lingering PRs, but sometimes it's just the way it is.

[gaaclarke]

@gaaclarke, I see that issue flutter/flutter#13937 is closed because it has been fixed on master. Do you know when it will hit stable?

I have a rough idea. I don't know if publicly we've promised a date for the next stable release. It may be later than we were originally expecting. It will be the next stable release though.

I see a few possible approaches:

• We add a note to the text that this is fixed on master and then remove when it hits stable
• We add a note that it will be stable in release 3.3.8 (or whatever)
• We leave this PR as is until it does actually hit stable

I'm not a fan of lingering PRs, but sometimes it's just the way it is.

It isn't bothering me beyond the stress of making sure it doesn't get forgotten. Any of those sounds fine to me, just let me know what works for you. I already am in this predicament because of the medium article I'm sitting on anyways.

[sfshaza2]

ok, I'll leave this for now. But I'm happy to review your article, @gaaclarke!

[gaaclarke]

ok, I'll leave this for now. But I'm happy to review your article, @gaaclarke!

Thanks shams, it's the one you already reviewed, not a new one =)

[sfshaza2]

closing