Added git disclaimer about dealing with ssh host checking

This disclaimer already exists in library/source_control/hg but not here.  It affects both and was discussed awhile back on: https://groups.google.com/forum/#!msg/ansible-project/q9G9xIdsRxo/XJhQzuS-qSoJ  However no PR was ever sent to fix the doc.
Merge pull request #3784 from drewblas/devel

Added git disclaimer about dealing with ssh host checking