api: Fix documentation of the format used for git date fields
They may contain negative UTC offsets, which is allowed by RFC3339, but not by ISO8601.
See the last paragraph of https://en.wikipedia.org/wiki/ISO_8601#Time_offsets_from_UTC for details
I also removed (in UTC) because it's not true.
Migrated from D6730 (view on Phabricator)