Clarify the start and end of scan intervals

Update the documentation of NetworkScanRequest to clarify
that the interval between scans is from the completion of
one scan to the start of another. This is the only possible
definition that doesn't possibly result in back-to-back
scans which never complete.

In the initial design of this API, the stated use case was
for scans where "interval" >> "scan duration". For that
use case, this clarification doesn't make a meaningful
difference; however, for the use case of long-duration
scans, the distinction prevents the issue stated above.

Bug: 139935383
Test: compilation (docstring-only change)
Change-Id: Ib8393110bfd3ea883045648ee7dac9c6e6a32d44