Bug fix list for iOS

I am new to iOS. Would like to seek for advice on where can one go search for list of bug (or improvement) made by iOS from v8.4 to the current one. Tried the search in this forum and Google without much success. Is there any Apple official list or other 3rd parties descriptions for that ?

The topics I am interested in are related to use of iPhone for Exchange email or application display. Any suggestion will be much appreciated. Thanks in advance.