Add specific Processing headings + minor editorial

[sideshowbarker]

This splits the “Link type "serviceworker"” ≫ “Processing” section into
two sections with separate headings:

• Processing the Link header
• Processing the link element

It also includes a minor editorial change to add the word “or” in a
place where lack of it makes the text ambiguous; this sentence:

When a serviceworker link’s link element is inserted into a document,
a serviceworker link is created on a link element that is already in a
document tree, or the href or scope attributes of the link element of a
serviceworker link is changed, the user agent should run these steps

...could be read as saying:

When a serviceworker link’s link element is inserted into a document,
then a serviceworker link is created on a link element...

When what it actually seems to mean is:

When a serviceworker link’s link element is inserted into a document,
or a serviceworker link is created on a link element...

That “or” is somewhat implied from the rest of the sentence, but this
this change just inserts an explicit “or” to make it more clear.

[sideshowbarker]

Marked as non-substantive for IPR from ash-nazg.

[jungkees]

LGTM.

a minor editorial change to add the word “or”

I think the original sentence was still okay to understand within the context but agree to this change with no strong opinion. /cc @mkruisselbrink

[jungkees]

Thanks @sideshowbarker!

[mkruisselbrink]

Thanks!