Include terminal accessibility in known issues

[aefernandes]

In the accessibility docs page, we should include some mention of the status of the terminal.

Currently the only mention is that "the Integrated Terminal panel respects the Tab trapping mode and can be toggled with ⌃⇧M."

Perhaps add something that states that screen reader compatibility is a known issue and/or comments on which screen readers we primarily design for (i.e. NVDA) so that users are aware in advance.

Also see microsoft/vscode#8339

cc @Tyriar