[0kaba0hub]

Documents the yarilo PR that gives virtual mailboxes \Recent.

• protocols/imap.md, Recent messages: a virtual mailbox has recent messages of its own, and a + folder gives up its recent messages to that SELECT.
• features/virtual.md, +Folder: what + takes, arrivals while the virtual mailbox is selected, folders without +, and the virtual mailbox's own recent.
• architecture/parity.md, the virtual mailboxes row: the departure is gone.

npm run docs:build and check-anchors pass locally.

[0kaba0hub]

APPROVED at 4b56de0, to merge after yarilo#2386. The +Folder text, the Recent messages bullet and the parity row now state what #2386 does: a virtual mailbox has recent messages of its own, + hands a folder's to the virtual SELECT and to its polls, a folder without + keeps its own. Build green.