Summary:
In rare cases seeing failures like this

```
[ RUN      ] DBWriteTestInstance/DBWriteTest.LockWALInEffect/2
db/db_write_test.cc:653: Failure
Put("key3", "value")
Corruption: Not active
```

in a test with no explicit threading. This is likely because of the unpredictability of background auto-resume. I didn't really know this feature, in part because DB::Resume() was undocumented. So I believe I have fixed the test and documented the API function.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11290

Test Plan: 1000s of stress runs of the test with gtest-parallel

Reviewed By: anand1976

Differential Revision: D43984583

Pulled By: pdillinger

fbshipit-source-id: d30dec120b4864e193751b2e33ff16834d313db3