doc: document url `slashes` property

Slashes should be documented, because 3rd-party protocols -- those
postfixed with `://` -- would incorrectly `format` and `parse` if they
didn't set/get the `slashes` option.

Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>
Merge remote-tracking branch 'upstream/v0.10' into v0.10