Documentation : Clarify Response Time or Elapsed Time

**Sebb** ([Bug 55168](https://bz.apache.org/bugzilla//show_bug.cgi?id=55168&redirect=false)):
The JMeter documentation sometimes refers to Response Time and sometimes to Elapsed Time.

I suspect the two are equivalent as far as JMeter is concerned, so we should consider standardising on a single name. However this may be tricky without breaking documentation links etc.

Or we could just update the Glossary to state that Response Time is the same as Elapsed Time (if indeed that is the case).

OS: All