Doc about PID pressure condition. (#8211)

* Doc about PID pressure condition.

Signed-off-by: Da K. Ma <klaus1982.cn@gmail.com>

* "so" -> "too"

docs/concepts/architecture/nodes.md

@@ -50,6 +50,7 @@ The `conditions` field describes the status of all `Running` nodes.
 | `OutOfDisk`    | `True` if there is insufficient free space on the node for adding new pods, otherwise `False` |
 | `Ready`        | `True` if the node is healthy and ready to accept pods, `False` if the node is not healthy and is not accepting pods, and `Unknown` if the node controller has not heard from the node in the last 40 seconds |
 | `MemoryPressure`    | `True` if pressure exists on the node memory -- that is, if the node memory is low; otherwise `False` |
+| `PIDPressure`    | `True` if pressure exists on the processes -- that is, if there are too many processes on the node; otherwise `False` |
 | `DiskPressure`    | `True` if pressure exists on the disk size -- that is, if the disk capacity is low; otherwise `False` |
 | `NetworkUnavailable`    | `True` if the network for the node is not correctly configured, otherwise `False` |
 | `ConfigOK`    | `True` if the kubelet is correctly configured, otherwise `False` |