JavaDoc of TemperaryFolder: folder not guaranteed to be deleted

This adjusts the JavaDoc of `TemporaryFolder` to make it clear that the folders created by the rule are not guaranteed to be deleted after the test finishes, cf. discussion in #1001 and #616.
Nits:
- end sentence with a period
- blank line (only a *) on the next line
- following line should be '<p>Example of usage:'

    * No exception will be thrown in case the deletion fails.
    *
    * <p>Example of usage:
    * <pre>