doc: clarify assert.fail doc
doc: clarify assert.fail doc

PR-URL: https://github.com/nodejs/node/pull/4186
Reviewed-By: Michaël Zasso <dc852f7f42c586356f6be32b8780273088b3bded@gmail.com>
Reviewed-By: Roman Klauke <d817af1499bb518fdf015b00b45f20c6b53728ce@gmail.com>
Reviewed-By: Minwoo Jung <8ed4d68b4c6f320972a8484fc5a67b9b0188ed87@gmail.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node