The default value of the `len` parameter is zero and it is included in
the documenetation.
This patch also has examples of how `ftruncate` can be used.
PR-URL: https://github.com/nodejs/node/pull/7648
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>