env: Fix documentation for env_get_f()

This function actually returns:
- the number of bytes written into @buf excluding the terminating
  NULL-byte, if there was enough space in @buf
- the number of bytes written into @buf including the terminating
  NULL-byte, if there wasn't enough space in @buf
- -1 if the variable is not found

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>