docs: Update description on size_gb in disk field (googleapis#11615)

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: gcf-owl-bot[bot]

packages/google-cloud-batch/google/cloud/batch_v1/types/job.py

@@ -505,15 +505,22 @@ class Disk(proto.Message):
             size_gb (int):
                 Disk size in GB.
 
-                For persistent disk, this field is ignored if
-                ``data_source`` is ``image`` or ``snapshot``. For local SSD,
-                size_gb should be a multiple of 375GB, otherwise, the final
-                size will be the next greater multiple of 375 GB. For boot
-                disk, Batch will calculate the boot disk size based on
-                source image and task requirements if you do not speicify
-                the size. If both this field and the boot_disk_mib field in
-                task spec's compute_resource are defined, Batch will only
-                honor this field.
+                **Non-Boot Disk**: If the ``type`` specifies a persistent
+                disk, this field is ignored if ``data_source`` is set as
+                ``image`` or ``snapshot``. If the ``type`` specifies a local
+                SSD, this field should be a multiple of 375 GB, otherwise,
+                the final size will be the next greater multiple of 375 GB.
+
+                **Boot Disk**: Batch will calculate the boot disk size based
+                on source image and task requirements if you do not speicify
+                the size. If both this field and the ``boot_disk_mib`` field
+                in task spec's ``compute_resource`` are defined, Batch will
+                only honor this field. Also, this field should be no smaller
+                than the source disk's size when the ``data_source`` is set
+                as ``snapshot`` or ``image``. For example, if you set an
+                image as the ``data_source`` field and the image's default
+                disk size 30 GB, you can only use this field to make the
+                disk larger or equal to 30 GB.
             disk_interface (str):
                 Local SSDs are available through both "SCSI"
                 and "NVMe" interfaces. If not indicated, "NVMe"

packages/google-cloud-batch/google/cloud/batch_v1alpha/types/job.py

@@ -624,15 +624,22 @@ class Disk(proto.Message):
             size_gb (int):
                 Disk size in GB.
 
-                For persistent disk, this field is ignored if
-                ``data_source`` is ``image`` or ``snapshot``. For local SSD,
-                size_gb should be a multiple of 375GB, otherwise, the final
-                size will be the next greater multiple of 375 GB. For boot
-                disk, Batch will calculate the boot disk size based on
-                source image and task requirements if you do not speicify
-                the size. If both this field and the boot_disk_mib field in
-                task spec's compute_resource are defined, Batch will only
-                honor this field.
+                **Non-Boot Disk**: If the ``type`` specifies a persistent
+                disk, this field is ignored if ``data_source`` is set as
+                ``image`` or ``snapshot``. If the ``type`` specifies a local
+                SSD, this field should be a multiple of 375 GB, otherwise,
+                the final size will be the next greater multiple of 375 GB.
+
+                **Boot Disk**: Batch will calculate the boot disk size based
+                on source image and task requirements if you do not speicify
+                the size. If both this field and the ``boot_disk_mib`` field
+                in task spec's ``compute_resource`` are defined, Batch will
+                only honor this field. Also, this field should be no smaller
+                than the source disk's size when the ``data_source`` is set
+                as ``snapshot`` or ``image``. For example, if you set an
+                image as the ``data_source`` field and the image's default
+                disk size 30 GB, you can only use this field to make the
+                disk larger or equal to 30 GB.
             disk_interface (str):
                 Local SSDs are available through both "SCSI"
                 and "NVMe" interfaces. If not indicated, "NVMe"