clean up variable naming: threadNumber becomes forkNumber
[SUREFIRE-949] update documentation with new forkCount / reuseForks options

- also make ${surefire.threadNumber} replacement undocumented, as its naming is totally misleading and might give users wrong impressions
[SUREFIRE-949] update documentation with new forkCount / reuseForks options
[SUREFIRE-956] make forkNumber unique among concurrent surefire-executions in a parallel maven build