Clarify that weakMapMemoize is the default memoization function (v5)

[veksa]

Closes #753

I was reading through the API docs and hit the same confusion the issue describes: nowhere does it actually say that createSelector switched to weakMapMemoize by default in v5. You kind of have to already know it.

A couple of things made it worse. The createSelector examples pass memoize: weakMapMemoize and argsMemoize: weakMapMemoize explicitly, so it looks like you're supposed to configure that yourself — even though it's just the default now. And if you're coming from v4 (where the default was lruMemoize with a cache size of 1), there's no signal that the caching behavior changed under you.

While digging in I also noticed two pages that flat-out state the wrong default — createSelectorCreator and createStructuredSelector both say the args memoizer defaults to lruMemoize, which contradicts the source and the v5 summary.

So this PR:

• Adds a short note at the top of createSelector explaining that memoize and argsMemoize both default to weakMapMemoize since v5, how that differs from the old lruMemoize cache-of-1 behavior, why it helps (effectively unbounded, identity-keyed cache — no more "cache size of 1" surprise when a selector is called with many different args), the memory tradeoff (entries live as long as their args are referenced, then get GC'd), and that you don't need to pass these options unless you want to override them.
• Adds a note on the weakMapMemoize page saying it's the default now, and that the examples there pass memoize/argsMemoize explicitly only to show the behavior — a plain createSelector(...) already uses it.
• Fixes the lruMemoize → weakMapMemoize default in the createSelectorCreator and createStructuredSelector type-parameter tables.

Docs only. I double-checked the real defaults against src/createSelectorCreator.ts (argsMemoize defaults to weakMapMemoize, and the exported createSelector is built with it). No example source files changed, so nothing needs regenerating, and everything passes Prettier.

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