Add comments for region mapping and instance deletion process

Clarified how project IDs correlate to regions and added a note to address improvements once region data is included in the project list. Also provided context on instance deletion flow, explaining the need to wait for disk unlock before retries.

sky/provision/nebius/utils.py

@@ -41,6 +41,10 @@ def get_project_by_region(region: str) -> str:
     # project per region, and projects cannot be created at this time.
     # The region is determined from the project ID using a region-specific
     # identifier embedded in it.
+    # Project id looks like project-e00xxxxxxxxxxxxxx where
+    # e00 - id of region 'eu-north1'
+    # e01 - id of region 'eu-west1'
+    # TODO(SalikovAlex): fix when info about region will be in projects list
     # https://docs.nebius.com/overview/regions
     for project in projects.items:
         if region == 'eu-north1' and project.metadata.id[8:11] == 'e00':
@@ -284,6 +288,8 @@ def remove(instance_id: str) -> None:
     service.delete(
         nebius.compute().DeleteInstanceRequest(id=instance_id)).wait()
     retry_count = 0
+    # The instance begins deleting and attempts to delete the disk.
+    # Must wait until the disk is unlocked and becomes deletable.
     while retry_count < nebius.MAX_RETRIES_TO_DISK_DELETE:
         try:
             service = nebius.compute().DiskServiceClient(nebius.sdk())