Document changes to OTel propagator for v7.0.0

Follow up task to #5319 - need accompanying docs.

Since we don't have versioned docs, we can add docs changes noting this will be the behaviour starting with the next major, like this: https://github.com/getsentry/sentry-docs/pull/15295/changes

_Originally posted by @adinauer in https://github.com/getsentry/sentry-dotnet/pull/5319#pullrequestreview-5131082674_

Start by reviewing the propagator changes in issue #5319 and the linked sentry-docs pull request example. Add accompanying documentation describing the behavior that will begin with the next major release. Done means the OTel propagator change has a clear note in the appropriate documentation.

Written by the indexing model from the issue text.