doc: clarify slashes-appending in url module

PR-URL: #9731
Ref: #9521
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/url.md

@@ -154,10 +154,11 @@ The formatting process operates as follows:
  [`Error`][] is thrown.
 * For all string values of `urlObject.protocol` that *do not end* with an ASCII
  colon (`:`) character, the literal string `:` will be appended to `result`.
-* If either the `urlObject.slashes` property is true, `urlObject.protocol`
- begins with one of `http`, `https`, `ftp`, `gopher`, or `file`, or
- `urlObject.protocol` is `undefined`, the literal string `//` will be appended
- to `result`.
+* If either of the following conditions is true, then the literal string `//`
+ will be appended to `result`:
+ * `urlObject.slashes` property is true;
+ * `urlObject.protocol` begins with `http`, `https`, `ftp`, `gopher`, or
+ `file`;
 * If the value of the `urlObject.auth` property is truthy, and either
  `urlObject.host` or `urlObject.hostname` are not `undefined`, the value of
  `urlObject.auth` will be coerced into a string and appended to `result`