Fix misspellings.
Drop the heap and RAM used from _cat/nodes and add the RAM percentage.

In this view you never care about the actual heap used bytes; you only
want to know that your max is set to what you meant and what
percentage you're currently using.

Closes #4151.
Add full_id param to support displaying entire node ID.
Add _cat/help (halp!)
Add load average to _cat/nodes.
First pass at cat docs.