Document required field on spec site

The [SDF spec documentation](http://sdformat.org/spec) displays a "required" field for each element and attribute. The site shows values like `*` and `+` for the required field. Navigating through that documentation, I didn't find a way to tell what each of these symbols mean. I had to look through the API documentation to find the information:

https://github.com/osrf/sdformat/blob/f59a342469bd9c71bd2dc5b325d777164375b06a/include/sdf/Element.hh#L102-L108

It would be good to display this information on the spec site so users can easily tell what it means. I think it could be placed near the "Tree Key" on the top-right corner.

This issue was motivated by [this question](https://community.gazebosim.org/t/sdformat-spec-required-values/816).

Start with the SDF spec site and compare its displayed required-field symbols with the definitions in include/sdf/Element.hh at lines 102-108. Add an explanation near the “Tree Key” so users can understand `*` and `+`; it is done when the meanings are visible on the spec documentation page.

Written by the indexing model from the issue text.