Discussion of the 2nd case is missing: User-defined conversions _from_ values of a base class

The page lists a trivial and easy to understand case. However a more realistic and useful case is not discussed at all: User-defined conversions _from_ values of a base class are not allowed. It would be nice to discuss why, and to provide reasonable workarounds!

In particular with emphasis on the fact that the base class may not provide convenience conversion methods to derived classes for at least 2 reasons:
- The fact that base classes may be in other assemblies
- The fact that derived classes may introduce type parameters that do not exist on the base class

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#### Document Details

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* ID: eb04a5ff-461e-a70d-5e7c-20333e93c683
* Version Independent ID: d1a76803-3482-678d-b6b4-dd50b4d80fd8
* Content: [Compiler Error CS0553](https://docs.microsoft.com/en-us/dotnet/csharp/misc/cs0553?f1url=%3FappId%3Droslyn%26k%3Dk(CS0553))
* Content Source: [docs/csharp/misc/cs0553.md](https://github.com/dotnet/docs/blob/main/docs/csharp/misc/cs0553.md)
* Product: **dotnet-csharp**
* Technology: **csharp-diagnostics**
* GitHub Login: @BillWagner
* Microsoft Alias: **wiwagn**