ports/rx*/readme_threadx.txt: Update implementation information of in…

[TakeoTakahashi2020]

Hello, I checked the latest implementation about interrupt related information and update the readme_threadx.txt for RXv1/v2/v3. It would be appreciated if you could double-check and update. Thanks.

[ghost]

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:x: TakeoTakahashi2020 sign now
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[goldscott]

Thanks @TakeoTakahashi2020! We will update the readmes internally and they will be available in our next release, scheduled for October. There is a release at the end of this month, but it is too late to make any changes now.

[TakeoTakahashi2020]

@goldscott Thank you for picking up my PR. I understand the release schedule.

[goldscott]

I'll close this PR. We have updated the readmes internally and they will be available in the next release in a few months.

[TakeoTakahashi2020]

@goldscott OK, Thanks.