What is a "thread-local" mutex?

Change-Id: Idb0ca4bcefea4c1927e8238d3371c9044103239f
Reviewed-on: https://gerrit.libreoffice.org/6651
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>