Add further information to the IdleTuning content

[SueChaplain]

Subject: https://www.eclipse.org/openj9/docs/xxidletuningminidlewaittime/

Suggestion: From the slack channel:

"The earliest JVM detects idle state is after it has been idle for around 1 minute (but it is subject to change and it is not exposed to users). So any value less than 1 minute passed to -XX:IdleTuningMinIdleWaitTime gets rounded to 1 minute. Also, because of the way OpenJ9 currently detects its idle state, it may take anywhere around 'n' to 'n+100' secs to detect idle state where 'n' is the value specified to XX:IdleTuningMinIdleWaitTime option."

"When OpenJ9 releases memory pages during idle state, it does not modify internal GC structures which keep track of committed pages. It just de-commits the memory pages to OS using madvise() call. That's the reason Committed value does not reduce while RSS usage goes down during idle state. "