MessageDifferencer::StreamReporter Documentation Update

[acminor]

What version of protobuf and what language are you using? Version: v3.17.3 Language: C++

What operating system (Linux, Windows, ...) and version? Linux

What runtime / compiler are you using (e.g., python version or gcc version) g++ (Ubuntu 10.3.0-1ubuntu1~20.10) 10.3.0

What did you do? Steps to reproduce the behavior:

1. Create a MessageDifferencer
2. Create a StreamReporter with OstreamOutputStream(&std::cout)
3. Attach StreamReporter to MessageDifferencer
4. Use MessageDifferencer multiple times (with non-equal messages)
5. Allow StreamReporter to be deleted
6. See error for protobuf exception backup can only be called after next

What did you expect to see

No error.

I believe the documentation should mention that StreamReporter should be called with a new StreamReporter each time. Also, it appears that destructing the OstreamOutputStream in the StreamReporter is dangerous in that it destroys the internal Printer object which will throw an exception if it cannot call backup. This does not appear to be caught resulting in exceptions being bubbled up from a destructor.

What did you see instead?

Sorry I do not have the log. I can recreate it if needed.

Anything else we should know about your project / environment

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