[css-content-3] Add missing <content-list> sub-productions

[cdoublev]

Fixes #7368.

<content-list> currently expands to <string> | contents | <image> | <counter> | <quote> | <target> | <leader()>.

But according to the table of contents, it is missing <string()> and <content()> (annotations added, starting with ->):

2. <content-list> Values and Functions
   1. String -> <string>
   2. <image>
   3. Element Content -> contents
   4. Quotes -> <quote>
   5. Leaders -> <leader()>
   6. Cross references and the target-* functions -> <target>
   7. Named strings -> <string()>, <content()>

The proposed syntax includes <string()> and <content()>, and have its sub-productions sorted according to the table of contents (which makes it easier to go back and forth between the definitions of <content-list> and its sub-productions).

Please correct me if I am wrong, @faceless2: this syntax allows to remove the paragraphs related to <content-list> in CSS GCPM 3, below the string-set property definition table, and to reference <content-list> from CSS Content 3 instead, noting that <attr()>, as defined in CSS Values 5, can be used in any property declaration value.

content-list expands to one or more of the following values, in any order.

content-list = [ <string> | <counter()> | <counters()> | <content()> | attr(<identifier>) ]+

<string>: A string, as defined in [CSS21]
<counter()>: A counter() function, as described in [CSS21].
<counters()>: A counters() function, as described in [CSS21].
content(): The content() function, described below.
attr(<identifier>): Returns the string value of the attribute <identifier>, as defined in [CSS-VALUES-3]

[w3cbot]

svgeesus marked as non substantive for IPR from ash-nazg.