Document why quota is signed

[TheOneRing]

Why is quota signed.
If it is on purpose please explain the meaning of possible negative values.

libre-graph-api/api/openapi-spec/v1.0.yaml

Line 2582 in 354fb12

format: int64

[dschmidt]

Just sticking to the upstream spec, I assume: https://learn.microsoft.com/de-de/graph/api/resources/quota?view=graph-rest-1.0