Improve System.ComponentModel.BackgroundWorker-related documentation

# Improve System.ComponentModel.BackgroundWorker-related documentation

## General

See PR dotnet/docs#4548

## Issues with Existing Topics

The documentation for all [System.ComponentModel.BackgroundWorker](https://docs.microsoft.com/en-us/dotnet/api/system.componentmodel.backgroundworker?view=netframework-4.7.1) types is lacking in detail and in need of revision. This includes the following types:

- [BackgroundWorker class](https://docs.microsoft.com/dotnet/api/system.componentmodel.backgroundworker?view=netframework-4.7.1)
- [DoWorkEventHandler delegate](https://docs.microsoft.com/dotnet/api/system.componentmodel.doworkeventhandler?view=netframework-4.7.1)
- [DoWorkEventArgs class](https://docs.microsoft.com/dotnet/api/system.componentmodel.doworkeventargs?view=netframework-4.7.1)
- [RunWorkerCompletedEventHandler delegate](https://docs.microsoft.com/dotnet/api/system.componentmodel.runworkercompletedeventhandler?view=netframework-4.7.1#Remarks)
- [RunWorkerCompletedEventArgs class](https://docs.microsoft.com/dotnet/api/system.componentmodel.runworkercompletedeventargs?view=netframework-4.7.1#Properties_)

See dotnet/docs#4548.

@Tanya-Solyanik @SimpleSamples