Cleanup and document how to use options extensions

Hi all.

From my understanding of how this piece of code works,
`DbContextOptions.WithExtension(TExtension instance)` is supposed to replace the provided extension, if present, and return a new options object.

However, the [the implementation](https://github.com/dotnet/efcore/blob/809fe7a219c99fbe4f3576ef42bd084d4a6ce056/src/EFCore/DbContextOptions%60.cs#L54) is obtaining the type to replace from the generic parameter. i.e `extensions[typeof(TExtension)]`. I believe the intended behavior would be to use the instance type, i.e `extension.GetType()`

Take this unit test as an example:

```csharp
[Fact]
public void DbContextOptions_WithExtension_ShouldReplaceByInstanceType()
{
DbContextOptions dbContextOptions = new DbContextOptions();

SqliteOptionsExtension extension = new SqliteOptionsExtension();
dbContextOptions = dbContextOptions.WithExtension(extension);

RelationalOptionsExtension replaced = extension.WithMigrationsAssembly("MyMigrations");

Assert.IsType(replaced);
// This will be a WithExtension
// but on runtime, the instance will be of SqliteOptionsExtension type.
var newops = dbContextOptions.WithExtension(replaced);

// This assertion fails
Assert.Single(newops.Extensions);
}
```

I would expect the last call to `WithExtension` to have replaced the original `SqliteOptionsExtension`. However, as can be seen [on the implementation](https://github.com/dotnet/efcore/blob/809fe7a219c99fbe4f3576ef42bd084d4a6ce056/src/EFCore/DbContextOptions%60.cs#L54) the type of the extension is being obtained from the generic argument instead of the provided instance type.

A test where the generic argument is the same as the instance type does succeed

```csharp
[Fact]
public void DbContextOptions_WithExtension_ShouldReplaceByInstanceType_UsingReflection()
{
DbContextOptions dbContextOptions = new DbContextOptions();

SqliteOptionsExtension extension = new SqliteOptionsExtension();
dbContextOptions = dbContextOptions.WithExtension(extension);

RelationalOptionsExtension replaced = extension.WithMigrationsAssembly("MyMigrations");

Assert.IsType(replaced);

var genericMethod = dbContextOptions.GetType()
.GetMethod(nameof(dbContextOptions.WithExtension))
.MakeGenericMethod(new[] { extension.GetType() });

var newops = (DbContextOptions)genericMethod.Invoke(dbContextOptions, new[] { replaced });

Assert.Single(newops.Extensions);
}
```

The reason why I have faced this issue is since I would like to modify the `RelationalOptionsExtension` from a different place than where the `UseSqlite` or similar extension method is executed.

I'm not sure what the expected behavior is supposed to be, and I know that these are internal EFCore APIs, however I would have expected `WithExtension` to use the instance type from the runtime instance instead of the generic constraint being applied.

### Include provider and version information

EF Core version: 5.0.4
Target framework: NET 5.0.

Thank you all very much for your time!
Leo.