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>

doc/api/fs.md

@@ -693,6 +693,12 @@ fs.exists('/etc/passwd', (exists) => {
 });
 ```
 
+**Note that the parameter to this callback is not consistent with other
+Node.js callbacks.** Normally, the first parameter to a Node.js callback is
+an `err` parameter, optionally followed by other parameters. The
+`fs.exists()` callback has only one boolean parameter. This is one reason
+`fs.access()` is recommended instead of `fs.exists()`.
+
 Using `fs.exists()` to check for the existence of a file before calling
 `fs.open()`, `fs.readFile()` or `fs.writeFile()` is not recommended. Doing
 so introduces a race condition, since other processes may change the file's
@@ -774,17 +780,18 @@ process.
 ## fs.existsSync(path)
 <!-- YAML
 added: v0.1.21
-deprecated: v1.0.0
 -->
 
-> Stability: 0 - Deprecated: Use [`fs.statSync()`][] or [`fs.accessSync()`][]
-> instead.
-
 * `path` {String | Buffer}
 
 Synchronous version of [`fs.exists()`][].
 Returns `true` if the file exists, `false` otherwise.
 
+Note that `fs.exists()` is deprecated, but `fs.existsSync()` is not.
+(The `callback` parameter to `fs.exists()` accepts parameters that are
+inconsistent with other Node.js callbacks. `fs.existsSync()` does not use
+a callback.)
+
 ## fs.fchmod(fd, mode, callback)
 <!-- YAML
 added: v0.4.7
@@ -2167,7 +2174,6 @@ The following constants are meant for use with the [`fs.Stats`][] object's
 [`Buffer`]: buffer.html#buffer_buffer
 [Caveats]: #fs_caveats
 [`fs.access()`]: #fs_fs_access_path_mode_callback
-[`fs.accessSync()`]: #fs_fs_accesssync_path_mode
 [`fs.appendFile()`]: fs.html#fs_fs_appendfile_file_data_options_callback
 [`fs.exists()`]: fs.html#fs_fs_exists_path_callback
 [`fs.fstat()`]: #fs_fs_fstat_fd_callback
@@ -2180,7 +2186,6 @@ The following constants are meant for use with the [`fs.Stats`][] object's
 [`fs.readFile`]: #fs_fs_readfile_file_options_callback
 [`fs.stat()`]: #fs_fs_stat_path_callback
 [`fs.Stats`]: #fs_class_fs_stats
-[`fs.statSync()`]: #fs_fs_statsync_path
 [`fs.utimes()`]: #fs_fs_futimes_fd_atime_mtime_callback
 [`fs.watch()`]: #fs_fs_watch_filename_options_listener
 [`fs.write()`]: #fs_fs_write_fd_buffer_offset_length_position_callback