Highlight non-recursion of `artifacts:exclude` in `.gitlab-ci.yml` keyword documentation

What does this MR do?

This MR updates the .gitlab-ci.yml Keyword reference documentation to point out the difference in file matching behaviour between artifact:paths and artifact:excludes (the former being recursive while the latter is not). This difference can lead to unexpected results when attempting to exclude the full contents of a directory using the same syntax you would use to include the full contents of a directory.

Related issues

#15122 (comment 524690377) gitlab-runner!2205 (closed)

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