Until gi-docgen is declared stable support either of them.
This effectively means that hand builds and CI builds will use
gi-docgen, but distro builds use gtk-doc-tools.
For instance, we can tell the user that UEFI UpdateCapsule is disabled in the
system firmware, or that efivarfs is not mounted. This is much better than
creating "dummy" devices which are really just hacks around the problem because
no better API existed. THe dummy devices cause as many problems as they solve.
Plugins have to set FWUPD_PLUGIN_FLAG_USER_WARNING if a warning should be shown
to the user, and only one warning will be shown of each failure type.
It is expected that GUI clients like gnome-software and gnome-firmware would use
this API to notify the user the localized message for why firmware updates are
not being shown.
Fixes https://github.com/fwupd/fwupd/issues/2456
