fs,doc: undeprecate existsSync

This has been dragged through various long discussions and has been
elevated to the CTC multiple times.

As noted in
https://github.com/nodejs/node/pull/7455#issuecomment-228961530,
while this API is still generally considered an anti-pattern, there are
still use-cases it is best suited for, such as checking if a git rebase
is in progress by looking if ".git/rebase-apply/rebasing" exists.

The general consensus is to undeprecate just the sync version, given
that the async version still has the "arguments order inconsistency"
problem.

The consensus at the two last CTC meetings this came up at was also
to undeprecate existsSync() but keep exists() deprecated.
See: https://github.com/nodejs/node/issues/8242 &
https://github.com/nodejs/node/issues/8330

(Description write-up by @Fishrock123)

Fixes: https://github.com/nodejs/node/issues/1592
Refs: https://github.com/nodejs/node/pull/4217
Refs: https://github.com/nodejs/node/pull/7455
PR-URL: https://github.com/nodejs/node/pull/8364



Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>