docs: add item 26, no way to tag from the UIHEADmaster

The user asked how to add a tag to a message and the answer is that you
cannot: every tagging action writes a hardcoded name, so archive, delete,
spam, flag and toggle_unread are the only tags reachable from the UI. For
an application whose purpose is organising mail by tag, that is a hole
worth its own item rather than being folded into item 25.

The plumbing is already there. tagSelected() takes arbitrary add and
remove lists, applyTagsToThreads() handles multi-row selections in one
query, undo works, and the completer already holds every tag in the
database, which is what makes completing the dialog's input the obvious
guard against creating 'shoppping' beside 'shopping'.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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