The solrModules and additionalLibs configurations should work for custom solr.xml configMap

If you have a custom `solr.xml` in a configMap, then the `solrModules` and `additionalLibs` configurations will not work, as they are obly applied when building a defualt solr.xml.

Perhaps we could document that if your custom solr.xml does not contain these tags or they are empty, then the operator is allowed to overwrite them:

```xml

```
or also if the default variable is set:
```xml
${solr.sharedLib:}
```

No file, test, or documentation entry point is named in the issue. Review the operator documentation for custom solr.xml ConfigMaps, then verify that it explains how empty sharedLib tags or the ${solr.sharedLib:} default variable interact with solrModules and additionalLibs; done when users can determine the supported configuration from the docs.

Written by the indexing model from the issue text.