fix(ndt7-protocol): clarify ElapsedTime semantics

[bassosimone]

This change is 

[robertodauria]

with a comment.

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spec/ndt7-protocol.md line 350 at r1 (raw file):

information at different times. Usually the times would be very close in
time but there could be tiny differences caused by collecting distinct
metrics from distinct threads (or goroutines).

I would not reference goroutines here as they are specific to Go.

[bassosimone]

@robertodauria done, thank you!