Update fs.markdown to account for correct uses of fs.exists()

[oconnore]

fs.exists() is not an anti-pattern in itself. It becomes an anti-pattern when used in a particular way. Checking the existence of a file is quite useful when, for example, checking filesystem based locks or status files (of course, grabbing a lock is the same problem, and should use fs.open() directly), and the documentation shouldn't be so narrow in its assumptions.

[vkurchatkin]

@oconnore fs.exists is an antipattern because it doesn't follow (error, result) convention. it probably should be deprecated at some point.

[oconnore]

@vkurchatkin ah, but an api inconsistency is different than what the documentation is saying. In that case, the documentation should describe the inconsistency (lack of err argument in the callback, need to use domains? Or does error = false?), and mention a correct work-around, perhaps using fs.stat()?

[vkurchatkin]

@oconnore it should, I guess. Actually fs.exists it's nothing more than fs.stat with false result if err is not falsey.

[jasnell]

Closing this here. A new PR has been opened in http://github.com/nodejs/node