icr-images.md: elaborate on refresh a tiny bit?

[covener]

In https://github.com/OpenLiberty/ci.docker/blob/main/docs/icr-images.md we have:

The images for the latest release and the last two quarterly releases are available and are refreshed regularly.

Would it be reasonable to elaborate inline there about what that means? IIUC it means possibly new runtimes layer, which could have new java or new UBI, but never (?) new liberty content

[leochr]

@covener we'll update the doc to clarify what is meant by refresh. Only fixes for Java and OS are included with image refresh

[leochr]

In addition to the above, we should also clarify what is meant by 'quarterly releases'. Instead use two most recent Liberty Continious Delivery (CD) releases ending in “.3”, “.6", “.9” or “.12" along with some version examples.

Reference: Liberty support policy: https://www.ibm.com/support/pages/single-stream-continuous-delivery-sscd-software-support-lifecycle-policy-websphere-liberty