Ramp-up period not accurate

@rabelenda ([Bug 65031](https://bz.apache.org/bugzilla//show_bug.cgi?id=65031&redirect=false)):
There seems to be a problem in calculation of ramp up period according to the documentation.

The documentation states:

"The ramp-up period tells JMeter how long to take to "ramp-up" to the full number of threads chosen. If 10 threads are used, and the ramp-up period is 100 seconds, then JMeter will take 100 seconds to get all 10 threads up and running. Each thread will start 10 (100/10) seconds after the previous thread was begun. If there are 30 threads and a ramp-up period of 120 seconds, then each successive thread will be delayed by 4 seconds."

But doing some simple tests, if I set ramp-up period to 10 seconds, and 1 thread, the thread starts without any delay (so, full number of threads is reached in 0 time). If I use 2 threads, then first thread starts immediately and second thread starts after 5 seconds (reaching full number of threads at 5 seconds).

In general it seems that if ramp-up period is set to X seconds, and there are Y threads, the time taken to start all the threads is X - X/Y.

This is not a major issue, but one that may confuse users and that requires additional calculation to get the proper desired ramp-up.

Severity: minor
OS: All