Improve documentation for additional volumes

[mcarroll1]

Hello,

I found it difficult to specify additional volumes in the podOptions.volumes values, specifically specifying a source for the volume. It appears @janhoy had a similar issue here: https://github.com/apache/solr-operator/issues/484. Reading that issue helped me determine the correct syntax, but it might be helpful to improve the documentation in this regard for posterity. The syntax seems to be specific to the SolrCloud resource, and not generic K8s.

I'm happy to submit documentation updates, but first: where is the best place to add such documentation? It might be too complex for the chart values doc. Perhaps it would be best in these docs? Another place?

Thanks!