Add information about StreamContent disposing the content stream

[paulomorgado]

The content stream used to initialize StreamContent is disposed when the StreamContent instance is disposed.

This should be mentioned on the documentation.

https://docs.microsoft.com/en-us/dotnet/api/system.net.http.streamcontent.-ctor
https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Net.Http/StreamContent.xml

[mairaw]

Thanks @paulomorgado. I've added this to our backlog but we also welcome PRs to fix this.