Fix the statement requiring relative URL strings to match an in-container resource #3093

Section

https://www.w3.org/TR/epub-34/#sec-container-iri

Describe the problem

The section contains the following statement (near the end, before the note):

In addition, all relative-URL-with-fragment strings [url] MUST, after parsing, be equal to the content URL of an existing file in the OCF abstract container.

This wording was mine (oops) and introduced in #2028 as part of the long and technical URL-related changes.

The problems:

• primarily, it is the URL with the fragment removed that must be equal to the content URL of an existing container file. We somehow missed that in our previous reviews;
• editorially, "after parsing" does not specify which base URL to use;
• editorially, "equal to" may refer to its definition in the URL standard.

Describe the fix or new feature you propose

Reword as:

In addition, all URL records represented by relative-URL-with-fragment strings [url] in the OCF abstract container MUST equal the content URL of an existing file in the OCF abstract container, with exclude fragments set to true.