[css-anchor-position-1]: some keywords missing from `position-area` section

[tuankiet65]

In the section explaining what the keywords in position-area means, some keywords that appear in the syntax definition are missing. Example: span-self-start, span-self-end, span-left, span-right, ... I'd just fix the spec myself, except when I was editing the spec to add in the missing keywords, I also discovered #11371, so I'll wait until that issue is resolved.