doc: add notes to child_process.fork() and .exec()

Adds notes about the difference to their POSIX counterparts.

PR-URL: #1718
Fixes: #224
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/child_process.markdown

@@ -586,6 +586,8 @@ if it runs longer than `timeout` milliseconds. The child process is killed with
 amount of data (in bytes) allowed on stdout or stderr - if this value is
 exceeded then the child process is killed.
 
+*Note: Unlike the `exec()` POSIX system call, `child_process.exec()` does not replace
+the existing process and uses a shell to execute the command.*
 
 ### child_process.execFile(file[, args][, options][, callback])
 
@@ -645,6 +647,9 @@ done with care and by default will talk over the fd represented an
 environmental variable `NODE_CHANNEL_FD` on the child process. The input and
 output on this fd is expected to be line delimited JSON objects.
 
+*Note: Unlike the `fork()` POSIX system call, `child_process.fork()` does not clone the
+current process.*
+
 ## Synchronous Process Creation
 
 These methods are **synchronous**, meaning they **WILL** block the event loop,