Update ref-guide output snippets to match new default wt

Solr 7.0 changed the default wt format from XML to JSON. Many of the API-output snippets from our documentation were never updated to accommodate this change. So as-is, the output won't match what users following along at home would get from running the commands. This could throw some newer Solr users.

This JIRA is created to resolve this inconsistency. Either by adding the "wt=xml" param to the API command corresponding to any XML snippets, or by switching the snippets over to all use JSON.