Clarify "acquisition time" in scans file

[tsalo]

In the description for the scans file, the specification says:

If acquisition time is included it should be under acq_time header.

It would helpful to specify that the acquisition time should correspond to the beginning of acquisition (i.e., when the first data point for the run was acquired).

NOTE: This issue stems from nipy/heudiconv#450 (comment).

[sappelhoff]

I agree that this should be clarified and it seems fairly uncontroversial to specfiy as "when the first data point for the run was acquired". PR welcome!