Add API docs for System.Data.DataReaderExtensions (#2772)

* Add API docs for System.Data.DataReaderExtensions

* Update xml/System.Data.Common/DbDataReader.xml

Co-Authored-By: Ron Petrusha <[email protected]>

* Update xml/System.Data.Common/DbDataReader.xml

Co-Authored-By: Ron Petrusha <[email protected]>

* Apply suggestions from code review

Feedback from @rpetrusha. Thanks!

Co-Authored-By: Ron Petrusha <[email protected]>

* Addressing some of the feedback

* Minor tweak

* Fixed Task.Task.Result

* Fixed broken cref and xref

* fixred broken cref

* Adding SequentialAccess remark

* Fix another instance of Task.Result

* Tweak remarks

* More async and consistency goodness

* Removed empty line

Author: divega