Clarify create --stats duration vs. wall time (fixes #3301) (#3303)

clarify create --stats duration vs. wall time, fixes #3301

docs/faq.rst

@@ -288,6 +288,14 @@ needs to be ascertained and fixed.
 issues. We recommend to first run without ``--repair`` to assess the situation.
 If the found issues and proposed repairs seem right, re-run "check" with ``--repair`` enabled.
 
+Why is the time elapsed in the archive stats different from wall clock time?
+----------------------------------------------------------------------------
+
+Borg needs to write the time elapsed into the archive metadata before finalizing
+the archive, compacting the segments, and committing the repo & cache. This means
+when Borg is run with e.g. the ``time`` command, the duration shown in the archive
+stats may be shorter than the full time the command runs for.
+
 Security
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