docs: clarify how to fully clear a selector's cache (#569)

[veksa]

Summary

Closes #569.

A memoized selector keeps two separate caches because of Reselect's
two-stage "cascading" memoization:

• the argsMemoize cache for the arguments passed to the output selector,
  attached to the selector itself (selector.clearCache());
• the memoize cache for the input selectors' results, attached to
  memoizedResultFunc (selector.memoizedResultFunc.clearCache()).

selector.clearCache() only clears the first one. This trips people up when
they try to reset a selector between tests and see stale memoized results — the
exact confusion reported in #569.

Note: the original type error from that issue was on v4. On v5 the types are
already correct — both .clearCache() accesses type-check on default and lru
selectors (verified with tsc). So the real gap was purely documentation; this
PR documents the behavior and adds a regression type test to lock it in.

Changes

Docs

• FAQ.mdx – add a note in "How do I test a selector?" explaining the two
  caches and showing how to fully reset a selector between tests.
• api/createSelector.mdx – add a "Clearing the cache" note under the Output
  Selector Fields table.

Types

• src/types.ts – expand the JSDoc on memoizedResultFunc and clearCache so
  the dual-cache behavior shows up in IDE hovers.
• typescript_test/argsMemoize.typetest.ts – add a type test asserting that
  clearCache / resultsCount / resetResultsCount exist on both a default
  selector and its memoizedResultFunc.

Testing

• tsc --noEmit -p typescript_test/tsconfig.json passes.

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