Update backend.py docstrings
Update backend.py docstrings

This patch converts all of backend.py to epydoc formatting.

Reviewed-by: imsnah

dimara/ganeti,dimara/ganeti,andir/ganeti,mbakke/ganeti,grnet/snf-ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,apyrgio/ganeti,leshchevds/ganeti,yiannist/ganeti,leshchevds/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,apyrgio/ganeti,mbakke/ganeti,onponomarev/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,bitemyapp/ganeti,leshchevds/ganeti,onponomarev/ganeti,bitemyapp/ganeti,andir/ganeti,apyrgio/snf-ganeti,grnet/snf-ganeti,mbakke/ganeti