Document meaning of pemWorkDirectory

**Jetty version(s)**
12

**Enhancement Description**
https://stackoverflow.com/a/77318094/14731 provides an excellent explanation of what ServerQuicConfiguration.pemWorkDirectory means and what it is used for. Please add a similar explanation to the `ServerQuicConfiguration` Javadoc.

Start by locating the ServerQuicConfiguration class and read the existing Javadoc for pemWorkDirectory. Compare it with the explanation in the linked Stack Overflow answer, then update the Javadoc so the directory's meaning and use are clear. Done means the documentation explains pemWorkDirectory without changing behavior.

Written by the indexing model from the issue text.