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--allow-local-commits was removed in 4a301c5f ("Remove the
--allow-local-commits flag from wpt-import") so stop recommending its usage.

Additionally, given the amount of export commits we have in
web-platform-tests it is likely that unauthenticated runs will fail, so
explain how to use the --credentials-json option in both wpt-export and
wpt-import.

Bug: 816390
Change-Id: I72d7bc6dcb80d0db372e9dac1176f68d1b812bcb
Reviewed-on: https://chromium-review.googlesource.com/937241Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#539116}