Fix documentation for accept_hostkey in GIT module

Upon reading the documentation for the new `accept_hostkey` option in the `git` module, I could not determine what kind of values it expected. After pulling down the source I noticed it had the same argument spec `accept_hostkey=dict(default='no', type='bool'),` as the bare option which is a `yes` or `no` choice. 

So this PR adjusts the documentation so that the default is correctly labeled as `no` and adds the `yes, no` choices. 

This is my first time looking at python so please go easy if my assumptions are incorrect. If it does not accept a `yes, no` choice right now, it should be updated to match bare, force, and update options on the git module.