Include Dath Math expression unit options in Solr Docs

Currently the only way to find the complete list of Data Math expression units is trial and error or looking in the code: https://github.com/apache/solr/blob/d8f57f084d59f2e4b5a74dbe772d6af09a21dbb8/solr/core/src/java/org/apache/solr/util/DateMathParser.java#L137.

For a better user experience, we should add these to the Solr ref docs as well.