Little confusing words and statements on this page

Little confusing words and statements. Example:
In common application models, only one thread at a time executes user code, which minimizes the need for thread safety. For this reason, the .NET class libraries are not thread safe by default.

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#### Document Details

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* ID: 32c0c68d-17d3-7df5-a390-83e8751cc2dc
* Version Independent ID: e6701fcd-872b-14a0-a86d-76d328832d60
* Content: [Managed Threading Best Practices](https://docs.microsoft.com/en-us/dotnet/standard/threading/managed-threading-best-practices)
* Content Source: [docs/standard/threading/managed-threading-best-practices.md](https://github.com/dotnet/docs/blob/main/docs/standard/threading/managed-threading-best-practices.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @BillWagner
* Microsoft Alias: **wiwagn**