Do not allow the starting port number of serviceNodePortRange to be changed to a smaller one.

The current document [1] describes how to extend “serviceNodePortRange” as follows:

1. To expand the node port range, enter the following command. Replace <port> with the largest port number in the new range.
~~~
$ oc patch network.config.openshift.io cluster --type=merge -p \
  '{
    "spec":
      { "serviceNodePortRange": "30000-<port>" }
  }'
~~~
[1] https://docs.openshift.com/container-platform/4.15/networking/configuring-node-port-service-range.html

However, as of now, changing the starting port number of “serviceNodePortRange” to a smaller one is not an error.
At this point, however, the process of changing the starting port number(30000) in "serviceNodePortRange" to a smaller one does not fail.

4.15

eg) 
Change the "serviceNodePortRange" from "30000-32768" to "29000-32768".

oc patch network.config.openshift.io cluster --type=merge -p '{ "spec": { "serviceNodePortRange": "29000-32768" } }'

no error

$ oc patch network.config.openshift.io cluster --type=merge -p '{ "spec": { "serviceNodePortRange": "29000-32768" } }'
network.config.openshift.io/cluster patched

$ oc get network -o yaml cluster
apiVersion: config.openshift.io/v1
kind: Network
...
spec:
  ...
  serviceNodePortRange: 29000-32768

should terminate with an error

I believe that the code should be changed to strengthen error checking to avoid unnecessary problems.
Alternatively, I believe that the documentation should be modified to clearly state that the starting port number cannot be changed, although this will not result in an error.