In Owl 2, reactivity was tracked per-component-instance: when state read by
a descendant or by slot content was mutated, the parent that owned the
state re-rendered as a whole and its `patched` hook fired. In Owl 3, signal
reads subscribe whichever component is currently rendering, so a parent
that merely holds a signal is no longer re-rendered just because a child or
slot reads it — and its `onPatched` no longer fires.

The hooks still work as documented (they fire when the component's own
bdom is patched), but the surrounding behavior changed enough that code
ported from Owl 2 can silently stop reacting. Documents the new scope and
points readers at `effect` for value-driven side effects.

- reference/component.md: scope subsection on `patched`, with the
  slot+signal example, plus a "what `patched` is for vs. when to reach
  for `effect`" subsection. `willPatch` cross-references the same rules.
- migration_owl2_to_owl3.md: new entry #22 with the same example, an
  explanation of why Owl 2 behaved differently, and an `onPatched` →
  `effect` migration recipe.