Highlight non-recursion of `artifacts:exclude`

The file-matching syntax of `artifacts:paths` and `artifacts:exclude`
seem (and are) similar, but `exclude` differs in that it does not
include directory contents recursively. This can lead to confusion when
attempting to exclude the contents of a subdirectory by matching that
subdirectory's path. Explain this caveat in the docs and add a relevant
example.

See https://gitlab.com/gitlab-org/gitlab/-/issues/15122#note_524690377
for an example of this issue in the wild.

doc/ci/yaml/README.md

@@ -3283,6 +3283,18 @@ artifacts:
     - binaries/**/*.o
 ```
 
+Unlike [`artifacts:paths`](#artifactspaths), `exclude` paths are not recursive. To exclude all of the contents of a directory, you can match them explicitly rather than matching the directory itself.
+
+For example, to store all files in `binaries/` but nothing located in the `temp/` subdirectory:
+
+```yaml
+artifacts:
+  paths:
+    - binaries/
+  exclude:
+    - binaries/temp/**/*
+```
+
 Files matched by [`artifacts:untracked`](#artifactsuntracked) can be excluded using
 `artifacts:exclude` too.