Improve code coverage extraction documentation

doc/ci/yaml/README.md

@@ -3283,6 +3283,11 @@ job1:
   coverage: '/Code coverage: \d+\.\d+/'
 ```
 
+The coverage is shown in the UI if at least one line in the job output matches the regular expression.
+If there is more than one matched line in the job output, the last line is used.
+For the matched line, the first occurence of `\d+(\.\d+)?` is the code coverage.
+Leading zeros are removed.
+
 ### `retry`
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/-/issues/3442) in GitLab 9.5.