flag_require_ac documentation: System requires external power source
Related to https://github.com/fwupd/fwupd/pull/2403 and https://github.com/fwupd/fwupd/issues/2345
This fixes the wording issue discussed in issue 2345 (see link above). A system that has the flag "require_ac" does not actually run on alternating current. The device runs on direct current but just needs to be plugged into an external power source / an outlet.