Fix cache:key documentation for the default value
Fix cache:key documentation for the default value

* Re-apply changes from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9666
* Update the wordings accordingly for the current revision
* Update the example to make it less confusing

iiet/iiet-git,dreampet/gitlab,dreampet/gitlab,iiet/iiet-git,dreampet/gitlab,axilleas/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,stoplightio/gitlabhq,mmkassem/gitlabhq,jirutka/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,axilleas/gitlabhq,dreampet/gitlab,mmkassem/gitlabhq,axilleas/gitlabhq,iiet/iiet-git,jirutka/gitlabhq,jirutka/gitlabhq