This is a bug. I've tagged your question accordingly.
total, in the wrapper object, should always be the length of the items array, after concatenating all pages of results.
When I run /2.2/notifications?pagesize=100&filter=!*L6X1NqVdlFobvlv, I get 4 pages of results, currently totaling 382 items:
total remains fixed at 77. This is clearly a bug, ...
This is a very minor bug in the documentation page.
The online doc page adds a convenient hyperlink to the post (The link part is not in the API's JSON response). That's a good thing.
Most of the time this is rock solid, but notifications span all Stack exchange sites and the API doc pages normally only expect one site at a time.
When the page is making ...
You can – sometimes. The default filter contains a question_id field, and according to the examples for the inbox_item type (you have to scroll down a bit) it is filled in some situations. I just tested it and the notification for this comment looks like this:
It should be populated for the new_answer type as well, but I couldn't test this quickly. It is ...