Miscellaneous JSON Facet API docs improvements

While working on SOLR-12965 I noticed a few minor issues with the JSON faceting ref-guide pages. Nothing serious, just a few annoyances. Tweaks include:

• missing/insufficient description of some params for Heatmap facets
• Weird formatting on "Domain Filters" example
• missing "fields"/"fl" in the "Parameters Mapping" table

Figured I'd just create a JIRA and fix these before I forgot about them