Update build check documentation

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Co-authored-by: Copilot <[email protected]>

Fixes #12660

### Context
The following assert fails:
`/mt /m` sometimes crashes with error:
```
Microsoft.Build.Framework.InternalErrorException: MSB0001: Internal MSBuild Error: Why are we trying to disconnect from a context that we already disconnected from?  Did we call DisconnectFromHost twice?
   at Microsoft.Build.Shared.ErrorUtilities.ThrowInternalError(String message, Exception innerException, Object[] args) in C:\Users\alinama\work\msbuild\src\Shared\ErrorUtilities.cs:line 69
   at Microsoft.Build.Shared.ErrorUtilities.VerifyThrow(Boolean condition, String unformattedMessage) in C:\Users\alinama\work\msbuild\src\Shared\ErrorUtilities.cs:line 195
   at Microsoft.Build.BackEnd.NodeProviderOutOfProcTaskHost.DisconnectFromHost(Int32 nodeId) in C:\Users\alinama\work\msbuild\src\Build\BackEnd\Components\Communications\NodeProviderOutOfProcTaskHost.cs:line 607
   at Microsoft.Build.BackEnd.TaskHostTask.Execute() in C:\Users\alinama\work\msbuild\src\Build\Instance\TaskFactories\TaskHostTask.cs:line 382
   at Microsoft.Build.BackEnd.TaskExecutionHost.Execute() in C:\Users\alinama\work\msbuild\src\Build\BackEnd\TaskExecutionHost\TaskExecutionHost.cs:line 636
```

### Changes Made
Changed `_nodeIdToPacketFactory` and `_nodeIdToPacketHandler` from
`Dictionary` to `ConcurrentDictionary`.

### Testing
Manually tested to make sure the error is not there.

### Notes

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Co-authored-by: Jan Provazník <[email protected]>