Add use case content for CI cache key
In the 12.5 Release Post, we identified that the documentation for cache:key:files doesn't include use-case content for why someone might use this feature. Additionally, the current content is very hard to understand.
Current content:
##### `cache:key:files`
If `cache:key:files` is added, one or two files must be defined with it. The cache `key`
will be a SHA computed from the most recent commits (one or two) that changed the
given files. If neither file was changed in any commits, the key will be `default`.
Questions:
- Why would I add cache:key:files? What's the result? What's the value?
- What are the "one or two files" that must be defined? Do I define them myself or are they automatically defined from the most recent commits?
- What happens if I use the default key?
See here for original feedback: gitlab-com/www-gitlab-com!34283 (comment 247130430)