Define conditions under which IOException is thrown

# Define conditions under which IOException is thrown

## General
The [IOException](https://docs.microsoft.com/en-us/dotnet/api/system.io.ioexception?view=netframework-4.7.1#Remarks) documentation needs to be improved to discuss the conditions under which an IOException are thrown.

## Issues with Existing Topics

The [IOException](https://docs.microsoft.com/en-us/dotnet/api/system.io.ioexception?view=netframework-4.7.1#Remarks) documentation treats IOException as a base class of a number of other exceptions (such as FileLoadException, FileNotFoundException, etc.). However, an IOException rather than a derived class exception is thrown under a number of circumstances. For example, an IOException is thrown when no disk space is available; by convention, we don't document this exception condition, since any operation that touches storage can throw the exception.

See PR dotnet/docs#4171

//cc @mairaw @ahman0

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