Document that retry does not catch inner timeout

[dduportal]

As per https://issues.jenkins.io/browse/JENKINS-51454?focusedCommentId=381283&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-381283

this PR modifies the documentation for the retry block to mention that

retry(3) {
    timeout(XXX) {
    // steps
    }
}

does not behave as expected.

Ref. jenkins-infra/docker-jenkins-weeklyci#205 where we tried it, but we discovered that it never retries when the timeout is reached.

This commit can be safely reverted when #81 would be merged.

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[dwnusbaum]

#144 is a proposal to make the snippet in the description behave as expected.