cat/nodes currently does not report any details related to file descriptors. This adds the current number in use, the maximum number available as well as their ratio (percentage) to cat/nodes as hidden-by-default metrics. In addition, this also adds current heap usage (as a non-percentage of ts max) and ram usage (as a non-percerntage of its max) to allow tools to provide more granularity.
Closes#7652
Adds a table with the exhaustive list of all available headers with a brief description (mostly from `org.elasticsearch.rest.action.cat.RestNodesAction`) so that people do not need to go searching for them in the code like I did, or search through `nodes?help`.