docs: clarify url.format documentation

[misterdjules]

The original documentation was slightly confusing. It seemed that the
list of items described the properties of the urlObj object, while it
was actually describing the formatting process. This change makes this
clearer.

Fixes #8796.

[misterdjules]

@chrisdickinson @trevnorris @orangemocha @cjihrig Could you please take a look?

[chrisdickinson]

This LGTM as-is.

Sidenote: it might be helpful to illustrate how these keys work in the docs. How would you feel about including the following diagram:

+-----------------------------------------------------------------------------------+
| href                                                                              |
+----------+--+-----------+--------------------+-----------------------------+------+
| protocol |* | auth      | host               | path                        | hash |
|          |  |           +-------------+------+----------+------------------+      |
|          |  |           | hostname    | port | pathname | search           |      |
|          |  |           |             |      |          +-+----------------+      |
|          |  |           |             |      |          | | query          |      |
" http:     //   user:pass@some.host.com:65535 /path/name ? search=1&continues#hash "
|          |  |           |             |      |          | |                |      |
+----------+--+-----------+-------------+------+----------+-+----------------+------+
(all spaces in the "" line should be ignored -- they're purely for formatting)
*: given by "slashes" key

Where keys higher in the diagram override all keys underneath them. This is borrowed from #8755, which introduced the path param -- my comment. If you'd like I can also clean this up and turn it into an SVG and include it in a subsequent PR.

[misterdjules]

@chrisdickinson I like the diagram a lot! I think it would make a great addition to the documentation.
I think the ASCII diagram already looks good, and I don't know how a SVG rendering would look like.

I suggest landing this PR as is and adding the diagram in another PR. Does it work for you?

[misterdjules]

Landed in 0603c83, thanks @chrisdickinson!