[Docs] Clarify how K8s resources and offers work

Author: un-def

docs/docs/concepts/backends.md

@@ -1060,7 +1060,64 @@ projects:
 
     Ensure you've created a ClusterRoleBinding to grant the role to the user or the service account you're using.
 
-> To learn more, see the [Lambda](../../examples/clusters/lambda/#kubernetes) and [Lambda](../../examples/clusters/crusoe/#kubernetes) examples.
+??? info "Resources and offers"
+    [Resources](../concepts/tasks.md#resources) specified in the run configuration are translated to Kubernetes
+    [requests and limits](https://kubernetes.io/docs/concepts/configuration/manage-resources-containers/#requests-and-limits) as follows:
+
+    - As with other backends, an exact value is translated to a range with the same lower and upper limits,
+      e.g., `cpu: 4` is the same as `cpu: 4..4`.
+    - The lower limit, if set, is used as a resource request, meaning that it is guaranteed that the container has at least the specified
+      amount of the resource.
+    - For resources other than `gpu`, the upper limit, if set, is used as a resource limit, meaning that the container is not allowed
+      to consume more resources than specified. If the upper limit is not set, the resource limit is also not set.
+    - For `gpu` resources, the upper limit is always ignored, and the resource limit is always set to the same value as the resource request,
+      that is, to the lower limit of the range.
+
+    For example, the following resources specification:
+
+    <div editor-title=".dstack.yml">
+
+    ```yaml
+    resources:
+      cpu: 32..64
+      memory: 1024GB
+      disk: 100GB..
+      gpu: nvidia:4..8
+    ```
+
+    </div>
+
+    is translated to:
+
+    | resource            | request  | limit     |
+    |---------------------|----------|-----------|
+    | `cpu`               | `32`     | `64`      |
+    | `memory`            | `1024Gi` | `1024Gi`  |
+    | `ephemeral-storage` | `100Gi`  | _not set_ |
+    | `nvidia.com/gpu`    | `4`      | `4`       |
+
+    In offers, `dstack` uses resource requests as offer's resources.
+    With the resources spec as in the example above, offers would look like the following,
+    even if nodes have more available resources:
+
+    ```
+     #  BACKEND         RESOURCES                                 INSTANCE TYPE  PRICE
+     1  kubernetes (-)  cpu=32 mem=1024GB disk=100GB H100:80GB:4  h100x8         $0
+    ```
+
+    As a consequence, if you specify `gpu: 0` or don't specify `gpu` at all (the default value is `0`),
+    you won't see GPU resources in offers, even with GPU nodes. The same is true for the `dstack offer` command,
+    to see available GPU models, you should specify a miminum amount of GPUs > 0:
+
+    <div class="termy">
+
+    ```shell
+    $ dstack offer --gpu 1
+    ```
+
+    </div>
+
+> To learn more, see the [Lambda](../../examples/clusters/lambda/#kubernetes) and [Crusoe](../../examples/clusters/crusoe/#kubernetes) examples.
 
 ### RunPod