Clarify tag object creation and link with tag reference creation

Hi,

just two minor additions to the [tags API page](http://developer.github.com/v3/git/tags/) that I think will help in understanding how tags are created (motivated by a [SO question](http://stackoverflow.com/questions/15672547/how-to-tag-a-commit-in-api-using-curl-command)):

1. give an example of the input for creating tag objects (matching the response example that was already there), and
2. explicitly link the creation of tag objects with the creation of tag references. 

Cheers!