Update out-of-date JSDoc, rename parameter '_path'

A couple of trivial changes in ember-metal:

(a) Update out-of-date parameter names in JSDoc for some functions,
(b) Rename the parameter `_path` back to `path`.  At some point an anonymous function was used there with a parameter called `path`, so the outer function's parameter was renamed to `_path`.  The anonymous function has since been removed, so I changed it back.