doc: grammar, clarity and links in timers doc

Added appropriate in-document links. Clarified a bit of
`setImmediate`, including a quick grammar fix (plural possessive
apostrophe).

PR-URL: https://github.com/nodejs/node/pull/5792


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>