doc: clarify child_process.execFile{,Sync} file arg

The changes to the file argument of execFile in #4504 make it appear
that execFile requires an absolute or relative path to the executable
file, when it also supports a filename which will be resolved using
$PATH.  Although the example makes this clear, assuming there isn't a
node binary in $CWD, it's easy to overlook.  This commit clarifies that
point.

It also updates the argument description for execFileSync to match,
since it was overlooked in #4504 and behaves identically.

PR-URL: https://github.com/nodejs/node/pull/5310
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/child_process.markdown

@@ -184,7 +184,7 @@ replace the existing process and uses a shell to execute the command.*
 
 ### child_process.execFile(file[, args][, options][, callback])
 
-* `file` {String} A path to an executable file
+* `file` {String} The name or path of the executable file to run
 * `args` {Array} List of string arguments
 * `options` {Object}
   * `cwd` {String} Current working directory of the child process
@@ -502,7 +502,7 @@ configuration at startup.
 
 ### child_process.execFileSync(file[, args][, options])
 
-* `file` {String} The filename of the program to run
+* `file` {String} The name or path of the executable file to run
 * `args` {Array} List of string arguments
 * `options` {Object}
   * `cwd` {String} Current working directory of the child process