glib glib-utils gdbus-codegen: only warn on doc failures

This is an intentional deviation from Gentoo upstream because
in CrOS document generation is flaky so we convert the errors
into warnings.

BUG=b/200325608,b/200325807
TEST=Presubmit
RELEASE_NOTE=None

cos-patch: security-moderate
Change-Id: If3c0f5dec436e556921d31117d6ae3f2e0bfbc67
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/3129460
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Commit-Queue: Manoj Gupta <manojgupta@chromium.org>
Tested-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/overlays/chromiumos-overlay/+/22674
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Main-Branch-Verified: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Meena Shanmugam <meenashanmugam@google.com>