doc: clarify description of assert.ifError()

This fixes a few typographical errors (comma splices and the like) and
clarifies the description of assert.ifError(). It also standardizes the
document on "inequality" rather than having both "inequality" and "non-
equality".

PR-URL: https://github.com/nodejs/node/pull/2941


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>