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>