Document Struct fields in CRD spec

[ytsarev]

Looking at https://doc.crds.dev/github.com/absaoss/k8gb/k8gb.absa.oss/Gslb/v1beta1@v0.7.2 it is visible that some parts of Gslb spec are totally undocumented or contain automatically generated operator-sdk documentation placeholders.

We need to update api/v1beta1/gslb_types.go with proper documentation comments.