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

`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.