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>

doc/api/url.markdown

@@ -19,6 +19,10 @@ string will not be in the parsed object. Examples are shown for the URL
 
     Example: `'http:'`
 
+* `slashes`: The protocol requires slashes after the colon
+
+    Example: true or false
+
 * `host`: The full lowercased host portion of the URL, including port
   information.
 
@@ -83,6 +87,9 @@ Take a parsed URL object, and return a formatted URL string.
     postfixed with `://` (colon-slash-slash).
   * All other protocols `mailto`, `xmpp`, `aim`, `sftp`, `foo`, etc will
     be postfixed with `:` (colon)
+* `slashes` set to `true` if the protocol requires `://` (colon-slash-slash)
+  * Only needs to be set for protocols not previously listed as requiring
+    slashes, such as `mongodb://localhost:8000/`
 * `auth` will be used if present.
 * `hostname` will only be used if `host` is absent.
 * `port` will only be used if `host` is absent.