Document how to handle long SSH keys being used as YAML keys

The YAML specification limits simple keys to being 1024 characters
in length. Since SSH keys are often longer than that, and since
the documentation provides examples using the SSH key as a YAML
key, demonstrate the proper way of specifying an YAML 'explicit'
key.
Merge pull request #7985 from kula/yaml-long-keys

Document how to handle long SSH keys being used as YAML keys