docs: Clarify that anchor is the Popover's origin, not the display side (#2537)

## Description

Closes #2534.

Users read the `anchor` option as "which side of the trigger the popover
shows on" and reported it as inverted. It actually uses GPUI's
corner-anchoring model, and the docs described it in a way that
reinforced the wrong mental model.

This PR only clarifies documentation, no behavior change. Instead of
explaining the mechanics, it uses a concrete image: imagine the popover
has a pointer tip (like a speech bubble's tail) — the anchor is where
that tip sits relative to the trigger (`Anchor::TopLeft` → the trigger's
top-left corner, `Anchor::BottomRight` → the bottom-right, and so on),
and the popover hangs off that point.

- Rewrite the `anchor()` doc comments on `Popover` and `HoverCard`.
- Update the Popover and HoverCard docs (en + zh-CN) with the same
explanation.
- Drop stale wording that implied two separate `Anchor` types.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>