Update copy in table of contents to describe the outline section.

[mtias]

Now that the tooltip itself is called "table of contents" it makes more sense to describe this one as "document outline" instead:

[aduth]

Code-wise 👍

[youknowriad]

It's still not clear to me what's "Document Outline" and what's "Table of Contents".
As I understand it:

• The table of contents is the list of titles
• The document outline is the table of contents + the counts (the whole dropdown)
• Also, there's the sidebar panel which right now is labeled as "Table of Contents"

According to the change in this PR, It might be the opposite though. So just wanted to ask for clarification and if we do the change in this PR, we should probably update the title in the sidebar and also merge #4309

[mtias]

Yes, it is kind of the opposite, it will stabilize soon enough :)