Updated the documentation to explain that when the maximum threads is > 1

Updated the documentation to explain that when the maximum threads is > 1

* glib/gthreadpool.c: Updated the documentation to explain that
when the maximum threads is > 1 the sort functionality is not 100%
accurate due to the ramdom nature of the scheduler choosing which
threads to execute. Fixes bug #334943.

* tests/threadpool-test.c: Disabled the debugging by default and
fixed the sort test to set the maximum threads to 1 to guarantee
the thread entry function is called in order.